NAME¶
Regexp::Grammars - Add grammatical parsing features to Perl 5.10 regexes
VERSION¶
This document describes Regexp::Grammars version 1.036
SYNOPSIS¶
use Regexp::Grammars;
my $parser = qr{
(?:
<Verb> # Parse and save a Verb in a scalar
<.ws> # Parse but don't save whitespace
<Noun> # Parse and save a Noun in a scalar
<type=(?{ rand > 0.5 ? 'VN' : 'VerbNoun' })>
# Save result of expression in a scalar
|
(?:
<[Noun]> # Parse a Noun and save result in a list
(saved under the key 'Noun')
<[PostNoun=ws]> # Parse whitespace, save it in a list
# (saved under the key 'PostNoun')
)+
<Verb> # Parse a Verb and save result in a scalar
(saved under the key 'Verb')
<type=(?{ 'VN' })> # Save a literal in a scalar
|
<debug: match> # Turn on the integrated debugger here
<.Cmd= (?: mv? )> # Parse but don't capture a subpattern
(name it 'Cmd' for debugging purposes)
<[File]>+ # Parse 1+ Files and save them in a list
(saved under the key 'File')
<debug: off> # Turn off the integrated debugger here
<Dest=File> # Parse a File and save it in a scalar
(saved under the key 'Dest')
)
################################################################
<token: File> # Define a subrule named File
<.ws> # - Parse but don't capture whitespace
<MATCH= ([\w-]+) > # - Parse the subpattern and capture
# matched text as the result of the
# subrule
<token: Noun> # Define a subrule named Noun
cat | dog | fish # - Match an alternative (as usual)
<rule: Verb> # Define a whitespace-sensitive subrule
eats # - Match a literal (after any space)
<Object=Noun>? # - Parse optional subrule Noun and
# save result under the key 'Object'
| # Or else...
<AUX> # - Parse subrule AUX and save result
<part= (eaten|seen) > # - Match a literal, save under 'part'
<token: AUX> # Define a whitespace-insensitive subrule
(has | is) # - Match an alternative and capture
(?{ $MATCH = uc $^N }) # - Use captured text as subrule result
}x;
# Match the grammar against some text...
if ($text =~ $parser) {
# If successful, the hash %/ will have the hierarchy of results...
process_data_in( %/ );
}
QUICKSTART CHEATSHEET¶
In your program...¶
use Regexp::Grammars; Allow enhanced regexes in lexical scope
%/ Result-hash for successful grammar match
Defining and using named grammars...¶
<grammar: GRAMMARNAME> Define a named grammar that can be inherited
<extends: GRAMMARNAME> Current grammar inherits named grammar's rules
Defining rules in your grammar...¶
<rule: RULENAME> Define rule with magic whitespace
<token: RULENAME> Define rule without magic whitespace
<objrule: CLASS= NAME> Define rule that blesses return-hash into class
<objtoken: CLASS= NAME> Define token that blesses return-hash into class
<objrule: CLASS> Shortcut for above (rule name derived from class)
<objtoken: CLASS> Shortcut for above (token name derived from class)
Matching rules in your grammar...¶
<RULENAME> Call named subrule (may be fully qualified)
save result to $MATCH{RULENAME}
<RULENAME(...)> Call named subrule, passing args to it
<!RULENAME> Call subrule and fail if it matches
<!RULENAME(...)> (shorthand for (?!<.RULENAME>) )
<:IDENT> Match contents of $ARG{IDENT} as a pattern
<\:IDENT> Match contents of $ARG{IDENT} as a literal
</:IDENT> Match closing delimiter for $ARG{IDENT}
<%HASH> Match longest possible key of hash
<%HASH {PAT}> Match any key of hash that also matches PAT
</IDENT> Match closing delimiter for $MATCH{IDENT}
<\_IDENT> Match the literal contents of $MATCH{IDENT}
<ALIAS= RULENAME> Call subrule, save result in $MATCH{ALIAS}
<ALIAS= %HASH> Match a hash key, save key in $MATCH{ALIAS}
<ALIAS= ( PATTERN )> Match pattern, save match in $MATCH{ALIAS}
<ALIAS= (?{ CODE })> Execute code, save value in $MATCH{ALIAS}
<ALIAS= 'STR' > Save specified string in $MATCH{ALIAS}
<ALIAS= 42 > Save specified number in $MATCH{ALIAS}
<ALIAS= /IDENT> Match closing delim, save as $MATCH{ALIAS}
<ALIAS= \_IDENT> Match '$MATCH{IDENT}', save as $MATCH{ALIAS}
<.SUBRULE> Call subrule (one of the above forms),
but don't save the result in %MATCH
<[SUBRULE]> Call subrule (one of the above forms), but
append result instead of overwriting it
<SUBRULE1>+ % <SUBRULE2> Match one or more repetitions of SUBRULE1
as long as they're separated by SUBRULE2
<SUBRULE1> ** <SUBRULE2> Same (only for backwards compatibility)
<SUBRULE1>* % <SUBRULE2> Match zero or more repetitions of SUBRULE1
as long as they're separated by SUBRULE2
In your grammar's code blocks...¶
$CAPTURE Alias for $^N (the most recent paren capture)
$CONTEXT Another alias for $^N
$INDEX Current index of next matching position in string
%MATCH Current rule's result-hash
$MATCH Magic override value (returned instead of result-hash)
%ARG Current rule's argument hash
$DEBUG Current match-time debugging mode
Directives...¶
<require: (?{ CODE }) > Fail if code evaluates false
<timeout: INT > Fail after specified number of seconds
<debug: COMMAND > Change match-time debugging mode
<logfile: LOGFILE > Change debugging log file (default: STDERR)
<fatal: TEXT|(?{CODE})> Queue error message and fail parse
<error: TEXT|(?{CODE})> Queue error message and backtrack
<warning: TEXT|(?{CODE})> Queue warning message and continue
<log: TEXT|(?{CODE})> Explicitly add a message to debugging log
<ws: PATTERN > Override automatic whitespace matching
<minimize:> Simplify the result of a subrule match
<context:> Switch on context substring retention
<nocontext:> Switch off context substring retention
DESCRIPTION¶
This module adds a small number of new regex constructs that can be used within
Perl 5.10 patterns to implement complete recursive-descent parsing.
Perl 5.10 already supports recursive=descent
matching, via the new
"(?<name>...)" and "(?&name)" constructs. For
example, here is a simple matcher for a subset of the LaTeX markup language:
$matcher = qr{
(?&File)
(?(DEFINE)
(?<File> (?&Element)* )
(?<Element> \s* (?&Command)
| \s* (?&Literal)
)
(?<Command> \\ \s* (?&Literal) \s* (?&Options)? \s* (?&Args)? )
(?<Options> \[ \s* (?:(?&Option) (?:\s*,\s* (?&Option) )*)? \s* \])
(?<Args> \{ \s* (?&Element)* \s* \} )
(?<Option> \s* [^][\$&%#_{}~^\s,]+ )
(?<Literal> \s* [^][\$&%#_{}~^\s]+ )
)
}xms
This technique makes it possible to use regexes to recognize complex,
hierarchical--and even recursive--textual structures. The problem is that Perl
5.10 doesn't provide any support for extracting that hierarchical data into
nested data structures. In other words, using Perl 5.10 you can
match
complex data, but not
parse it into an internally useful form.
An additional problem when using Perl 5.10 regexes to match complex data formats
is that you have to make sure you remember to insert whitespace-matching
constructs (such as "\s*") at every possible position where the data
might contain ignorable whitespace. This reduces the readability of such
patterns, and increases the chance of errors (typically caused by overlooking
a location where whitespace might appear).
The Regexp::Grammars module solves both those problems.
If you import the module into a particular lexical scope, it preprocesses any
regex in that scope, so as to implement a number of extensions to the standard
Perl 5.10 regex syntax. These extensions simplify the task of defining and
calling subrules within a grammar, and allow those subrule calls to capture
and retain the components of they match in a proper hierarchical manner.
For example, the above LaTeX matcher could be converted to a full LaTeX parser
(and considerably tidied up at the same time), like so:
use Regexp::Grammars;
$parser = qr{
<File>
<rule: File> <[Element]>*
<rule: Element> <Command> | <Literal>
<rule: Command> \\ <Literal> <Options>? <Args>?
<rule: Options> \[ <[Option]>+ % (,) \]
<rule: Args> \{ <[Element]>* \}
<rule: Option> [^][\$&%#_{}~^\s,]+
<rule: Literal> [^][\$&%#_{}~^\s]+
}xms
Note that there is no need to explicitly place "\s*" subpatterns
throughout the rules; that is taken care of automatically.
If the Regexp::Grammars version of this regex were successfully matched against
some appropriate LaTeX document, each rule would call the subrules specified
within it, and then return a hash containing whatever result each of those
subrules returned, with each result indexed by the subrule's name.
That is, if the rule named "Command" were invoked, it would first try
to match a backslash, then it would call the three subrules
"<Literal>", "<Options>", and
"<Args>" (in that sequence). If they all matched successfully,
the "Command" rule would then return a hash with three keys:
'Literal', 'Options', and 'Args'. The value for each of those hash entries
would be whatever result-hash the subrules themselves had returned when
matched.
In this way, each level of the hierarchical regex can generate hashes recording
everything its own subrules matched, so when the entire pattern matches, it
produces a tree of nested hashes that represent the structured data the
pattern matched.
For example, if the previous regex grammar were matched against a string
containing:
\documentclass[a4paper,11pt]{article}
\author{D. Conway}
it would automatically extract a data structure equivalent to the following (but
with several extra "empty" keys, which are described in
"Subrule results"):
{
'file' => {
'element' => [
{
'command' => {
'literal' => 'documentclass',
'options' => {
'option' => [ 'a4paper', '11pt' ],
},
'args' => {
'element' => [ 'article' ],
}
}
},
{
'command' => {
'literal' => 'author',
'args' => {
'element' => [
{
'literal' => 'D.',
},
{
'literal' => 'Conway',
}
]
}
}
}
]
}
}
The data structure that Regexp::Grammars produces from a regex match is
available to the surrounding program in the magic variable "%/".
Regexp::Grammars provides many features that simplify the extraction of
hierarchical data via a regex match, and also some features that can simplify
the processing of that data once it has been extracted. The following sections
explain each of those features, and some of the parsing techniques they
support.
Setting up the module¶
Just add:
use Regexp::Grammars;
to any lexical scope. Any regexes within that scope will automatically now
implement the new parsing constructs:
use Regexp::Grammars;
my $parser = qr/ regex with $extra <chocolatey> grammar bits /;
Note that you do not to use the "/x" modifier when declaring a regex
grammar (though you certainly may). But even if you don't, the module quietly
adds a "/x" to every regex within the scope of its usage. Otherwise,
the default
"a whitespace character matches exactly that
whitespace character" behaviour of Perl regexes would mess up your
grammar's parsing. If you need the non-"/x" behaviour, you can still
use the "(?-x)" of "(?-x:...)" directives to switch of
"/x" within one or more of your grammar's components.
Once the grammar has been processed, you can then match text against the
extended regexes, in the usual manner (i.e. via a "=~" match):
if ($input_text =~ $parser) {
...
}
After a successful match, the variable "%/" will contain a series of
nested hashes representing the structured hierarchical data captured during
the parse.
Structure of a Regexp::Grammars grammar¶
A Regexp::Grammars specification consists of a
start-pattern (which may
include both standard Perl 5.10 regex syntax, as well as special
Regexp::Grammars directives), followed by one or more rule or token
definitions.
For example:
use Regexp::Grammars;
my $balanced_brackets = qr{
# Start-pattern...
<paren_pair> | <brace_pair>
# Rule definition...
<rule: paren_pair>
\( (?: <escape> | <paren_pair> | <brace_pair> | [^()] )* \)
# Rule definition...
<rule: brace_pair>
\{ (?: <escape> | <paren_pair> | <brace_pair> | [^{}] )* \}
# Token definition...
<token: escape>
\\ .
}xms;
The start-pattern at the beginning of the grammar acts like the "top"
token of the grammar, and must be matched completely for the grammar to match.
This pattern is treated like a token for whitespace matching behaviour (see
"Tokens vs rules (whitespace handling)"). That is, whitespace in the
start-pattern is treated like whitespace in any normal Perl regex.
The rules and tokens are declarations only and they are not directly matched.
Instead, they act like subroutines, and are invoked by name from the initial
pattern (or from within a rule or token).
Each rule or token extends from the directive that introduces it up to either
the next rule or token directive, or (in the case of the final rule or token)
to the end of the grammar.
Tokens vs rules (whitespace handling)¶
The difference between a token and a rule is that a token treats any whitespace
within it exactly as a normal Perl regular expression would. That is, a
sequence of whitespace in a token is ignored if the "/x" modifier is
in effect, or else matches the same literal sequence of whitespace characters
(if "/x" is not in effect).
In a rule, most sequences of whitespace are treated as matching the implicit
subrule "<.ws>", which is automatically predefined to match
optional whitespace (i.e. "\s*").
Exceptions to this behaviour are whitespaces before a "|" or a code
block or an explicit space-matcher (such as "<ws>" or
"\s"), or at the very end of the rule)
You can explicitly define a "<ws>" token to change that default
behaviour. For example, you could alter the definition of
"whitespace" to include Perlish comments, by adding an explicit
"<token: ws>":
<token: ws>
(?: \s+ | #[^\n]* )*
But be careful not to define "<ws>" as a rule, as this will lead
to all kinds of infinitely recursive unpleasantness.
Per-rule whitespace handling
Redefining the "<ws>" token changes its behaviour throughout the
entire grammar, within every rule definition. Usually that's appropriate, but
sometimes you need finer-grained control over whitespace handling.
So Regexp::Grammars provides the "<ws:>" directive, which allows
you to override the implicit whitespace-matches-whitespace behaviour only
within the current rule.
Note that this directive does
not redefined "<ws>" within
the rule; it simply specifies what to replace each whitespace sequence with
(instead of replacign each with a "<ws>" call).
For example, if a language allows one kind of comment between statements and
another within statements, you could parse it with:
<rule: program>
# One type of comment between...
<ws: (\s++ | \# .*? \n)* >
# ...colon-separated statements...
<[statement]>+ % ( ; )
<rule: statement>
# Another type of comment...
<ws: (\s*+ | \#{ .*? }\# )* >
# ...between comma-separated commands...
<cmd> <[arg]>+ % ( , )
Note that each directive only applies to the rule in which it is specified. In
every other rule in the grammar, whitespace would still match the usual
"<ws>" subrule.
Calling subrules¶
To invoke a rule to match at any point, just enclose the rule's name in angle
brackets (like in Perl 6). There must be no space between the opening bracket
and the rulename. For example::
qr{
file: # Match literal sequence 'f' 'i' 'l' 'e' ':'
<name> # Call <rule: name>
<options>? # Call <rule: options> (it's okay if it fails)
<rule: name>
# etc.
}x;
If you need to match a literal pattern that would otherwise look like a subrule
call, just backslash-escape the leading angle:
qr{
file: # Match literal sequence 'f' 'i' 'l' 'e' ':'
\<name> # Match literal sequence '<' 'n' 'a' 'm' 'e' '>'
<options>? # Call <rule: options> (it's okay if it fails)
<rule: name>
# etc.
}x;
Subrule results¶
If a subrule call successfully matches, the result of that match is a reference
to a hash. That hash reference is stored in the current rule's own
result-hash, under the name of the subrule that was invoked. The hash will, in
turn, contain the results of any more deeply nested subrule calls, each stored
under the name by which the nested subrule was invoked.
In other words, if the rule "sentence" is defined:
<rule: sentence>
<noun> <verb> <object>
then successfully calling the rule:
<sentence>
causes a new hash entry at the current nesting level. That entry's key will be
'sentence' and its value will be a reference to a hash, which in turn will
have keys: 'noun', 'verb', and 'object'.
In addition each result-hash has one extra key: the empty string. The value for
this key is whatever substring the entire subrule call matched. This value is
known as the
context substring.
So, for example, a successful call to "<sentence>" might add
something like the following to the current result-hash:
sentence => {
"" => 'I saw a dog',
noun => 'I',
verb => 'saw',
object => {
"" => 'a dog',
article => 'a',
noun => 'dog',
},
}
Note, however, that if the result-hash at any level contains
only the
empty-string key (i.e. the subrule did not call any sub-subrules or save any
of their nested result-hashes), then the hash is "unpacked" and just
the context substring itself is returned.
For example, if "<rule: sentence>" had been defined:
<rule: sentence>
I see dead people
then a successful call to the rule would only add:
sentence => 'I see dead people'
to the current result-hash.
This is a useful feature because it prevents a series of nested subrule calls
from producing very unwieldy data structures. For example, without this
automatic unpacking, even the simple earlier example:
<rule: sentence>
<noun> <verb> <object>
would produce something needlessly complex, such as:
sentence => {
"" => 'I saw a dog',
noun => {
"" => 'I',
},
verb => {
"" => 'saw',
},
object => {
"" => 'a dog',
article => {
"" => 'a',
},
noun => {
"" => 'dog',
},
},
}
Turning off the context substring
The context substring is convenient for debugging and for generating error
messages but, in a large grammar, or when parsing a long string, the capture
and storage of many nested substrings may quickly become prohibitively
expensive.
So Regexp::Grammars provides a directive to prevent context substrings from
being retained. Any rule or token that includes the directive
"<nocontext:>" anywhere in the rule's body will not retain any
context substring it matches...unless that substring would be the only entry
in its result hash (which only happens within objrules and objtokens).
If a "<nocontext:>" directive appears
before the first
rule or token definition (i.e. as part of the main pattern), then the entire
grammar will discard all context substrings from every one of its rules and
tokens.
However, you can override this universal prohibition with a second directive:
"<context:>". If this directive appears in any rule or token,
that rule or token
will save its context substring, even if a global
"<nocontext:>" is in effect.
This means that this grammar:
qr{
<Command>
<rule: Command>
<nocontext:>
<Keyword> <arg=(\S+)>+ % <.ws>
<token: Keyword>
<Move> | <Copy> | <Delete>
# etc.
}x
and this grammar:
qr{
<nocontext:>
<Command>
<rule: Command>
<Keyword> <arg=(\S+)>+ % <.ws>
<token: Keyword>
<context:>
<Move> | <Copy> | <Delete>
# etc.
}x
will behave identically (saving context substrings for keywords, but not for
commands), except that the first version will also retain the global context
substring (i.e. $/{""}), whereas the second version will not.
Note that "<context:>" and "<nocontext:>" have
no effect on, or even any interaction with, the various result distillation
mechanisms, which continue to work in the usual way when either or both of the
directives is used.
Renaming subrule results¶
It is not always convenient to have subrule results stored under the same name
as the rule itself. Rule names should be optimized for understanding the
behaviour of the parser, whereas result names should be optimized for
understanding the structure of the data. Often those two goals are identical,
but not always; sometimes rule names need to describe what the data looks
like, while result names need to describe what the data means.
For example, sometimes you need to call the same rule twice, to match two
syntactically identical components whose positions give then semantically
distinct meanings:
<rule: copy_cmd>
copy <file> <file>
The problem here is that, if the second call to "<file>"
succeeds, its result-hash will be stored under the key 'file', clobbering the
data that was returned from the first call to "<file>".
To avoid such problems, Regexp::Grammars allows you to
alias any subrule
call, so that it is still invoked by the original name, but its result-hash is
stored under a different key. The syntax for that is: "<
alias=
rulename>". For example:
<rule: copy_cmd>
copy <from=file> <to=file>
Here, "<rule: file>" is called twice, with the first result-hash
being stored under the key 'from', and the second result-hash being stored
under the key 'to'.
Note, however, that the alias before the "=" must be a proper
identifier (i.e. a letter or underscore, followed by letters, digits, and/or
underscores). Aliases that start with an underscore and aliases named
"MATCH" have special meaning (see "Private subrule calls"
and "Result distillation" respectively).
Aliases can also be useful for normalizing data that may appear in different
formats and sequences. For example:
<rule: copy_cmd>
copy <from=file> <to=file>
| dup <to=file> as <from=file>
| <from=file> -> <to=file>
| <to=file> <- <from=file>
Here, regardless of which order the old and new files are specified, the
result-hash always gets:
copy_cmd => {
from => 'oldfile',
to => 'newfile',
}
List-like subrule calls¶
If a subrule call is quantified with a repetition specifier:
<rule: file_sequence>
<file>+
then each repeated match overwrites the corresponding entry in the surrounding
rule's result-hash, so only the result of the final repetition will be
retained. That is, if the above example matched the string
"foo.pl bar.py baz.php", then the result-hash would
contain:
file_sequence {
"" => 'foo.pl bar.py baz.php',
file => 'baz.php',
}
Usually, that's not the desired outcome, so Regexp::Grammars provides another
mechanism by which to call a subrule; one that saves
all repetitions of
its results.
A regular subrule call consists of the rule's name surrounded by angle brackets.
If, instead, you surround the rule's name with "<[...]>"
(angle
and square brackets) like so:
<rule: file_sequence>
<[file]>+
then the rule is invoked in exactly the same way, but the result of that
submatch is pushed onto an array nested inside the appropriate result-hash
entry. In other words, if the above example matched the same
"foo.pl bar.py baz.php" string, the result-hash would
contain:
file_sequence {
"" => 'foo.pl bar.py baz.php',
file => [ 'foo.pl', 'bar.py', 'baz.php' ],
}
This "listifying subrule call" can also be useful for non-repeated
subrule calls, if the same subrule is invoked in several places in a grammar.
For example if a cmdline option could be given either one or two values, you
might parse it:
<rule: size_option>
-size <[size]> (?: x <[size]> )?
The result-hash entry for 'size' would then always contain an array, with either
one or two elements, depending on the input being parsed.
Listifying subrules can also be given aliases, just like ordinary subrules. The
alias is always specified inside the square brackets:
<rule: size_option>
-size <[size=pos_integer]> (?: x <[size=pos_integer]> )?
Here, the sizes are parsed using the "pos_integer" rule, but saved in
the result-hash in an array under the key 'size'.
Parametric subrules¶
When a subrule is invoked, it can be passed a set of named arguments (specified
as
key"=>"
values pairs). This argument list is
placed in a normal Perl regex code block and must appear immediately after the
subrule name, before the closing angle bracket.
Within the subrule that has been invoked, the arguments can be accessed via the
special hash %ARG. For example:
<rule: block>
<tag>
<[block]>*
<end_tag(?{ tag=>$MATCH{tag} })> # ...call subrule with argument
<token: end_tag>
end_ (??{ quotemeta $ARG{tag} })
Here the "block" rule first matches a "<tag>", and the
corresponding substring is saved in $MATCH{tag}. It then matches any number of
nested blocks. Finally it invokes the "<end_tag>" subrule,
passing it an argument whose name is 'tag' and whose value is the current
value of $MATCH{tag} (i.e. the original opening tag).
When it is thus invoked, the "end_tag" token first matches 'end_',
then interpolates the literal value of the 'tag' argument and attempts to
match it.
Any number of named arguments can be passed when a subrule is invoked. For
example, we could generalize the "end_tag" rule to allow any prefix
(not just 'end_'), and also to allow for 'if...fi'-style reversed tags, like
so:
<rule: block>
<tag>
<[block]>*
<end_tag (?{ prefix=>'end', tag=>$MATCH{tag} })>
<token: end_tag>
(??{ $ARG{prefix} // q{(?!)} }) # ...prefix as pattern
(??{ quotemeta $ARG{tag} }) # ...tag as literal
|
(??{ quotemeta reverse $ARG{tag} }) # ...reversed tag
Note that, if you do not need to interpolate values (such as $MATCH{tag}) into a
subrule's argument list, you can use simple parentheses instead of
"(?{...})", like so:
<end_tag( prefix=>'end', tag=>'head' )>
The only types of values you can use in this simplified syntax are numbers and
single-quote-delimited strings. For anything more complex, put the argument
list in a full "(?{...})".
As the earlier examples show, the single most common type of argument is one of
the form:
IDENTIFIER "=>
$MATCH{"
IDENTIFIER"}". That is, it's a common
requirement to pass an element of %MATCH into a subrule, named with its own
key.
Because this is such a common usage, Regexp::Grammars provides a shortcut. If
you use simple parentheses (instead of "(?{...})" parentheses) then
instead of a pair, you can specify an argument using a colon followed by an
identifier. This argument is replaced by a named argument whose name is the
identifier and whose value is the corresponding item from %MATCH. So, for
example, instead of:
<end_tag(?{ prefix=>'end', tag=>$MATCH{tag} })>
you can just write:
<end_tag( prefix=>'end', :tag )>
Note that, from Perl 5.20 onwards, due to changes in the way that Perl parses
regexes, Regexp::Grammars does not support explicitly passing elements of
%MATCH as argument values within a list subrule (yeah, it's a very specific
and obscure edge-case):
<[end_tag(?{ prefix=>'end', tag=>$MATCH{tag} })]> # Does not work
Note, however, that the shortcut:
<[end_tag( prefix=>'end', :tag )]>
still works correctly.
Accessing subrule arguments more cleanly
As the preceding examples illustrate, using subrule arguments effectively
generally requires the use of run-time interpolated subpatterns via the
"(??{...})" construct.
This produces ugly rule bodies such as:
<token: end_tag>
(??{ $ARG{prefix} // q{(?!)} }) # ...prefix as pattern
(??{ quotemeta $ARG{tag} }) # ...tag as literal
|
(??{ quotemeta reverse $ARG{tag} }) # ...reversed tag
To simplify these common usages, Regexp::Grammars provides three convenience
constructs.
A subrule call of the form "<:"
identifier">"
is equivalent to:
(??{ $ARG{'identifier'} // q{(?!)} })
Namely:
"Match the contents of $ARG{'identifier'},
treating those contents as a pattern."
A subrule call of the form "<\:"
identifier">"
(that is: a matchref with a colon after the backslash) is equivalent to:
(??{ defined $ARG{'identifier'}
? quotemeta($ARG{'identifier'})
: '(?!)'
})
Namely:
"Match the contents of $ARG{'identifier'},
treating those contents as a literal."
A subrule call of the form "</:"
identifier">"
(that is: an invertref with a colon after the forward slash) is equivalent to:
(??{ defined $ARG{'identifier'}
? quotemeta(reverse $ARG{'identifier'})
: '(?!)'
})
Namely:
"Match the closing delimiter corresponding to the
contents of $ARG{'identifier'}, as if it were a
literal".
The availability of these three constructs mean that we could rewrite the above
"<end_tag>" token much more cleanly as:
<token: end_tag>
<:prefix> # ...prefix as pattern
<\:tag> # ...tag as a literal
|
</:tag> # ...reversed tag
In general these constructs mean that, within a subrule, if you want to match an
argument passed to that subrule, you use "<:"
ARGNAME">" (to match the argument as a pattern) or
"<\:"
ARGNAME">" (to match the argument as a
literal).
Note the consistent mnemonic in these various subrule-like interpolations of
named arguments: the name is always prefixed by a colon.
In other words, the "<:ARGNAME>" form works just like a
"<RULENAME>", except that the leading colon tells
Regexp::Grammars to use the contents of $ARG{'ARGNAME'} as the subpattern,
instead of the contents of "(?&RULENAME)"
Likewise, the "<\:ARGNAME>" and "</:ARGNAME>"
constructs work exactly like "<\_MATCHNAME>" and
"</INVERTNAME>" respectively, except that the leading colon
indicates that the matchref or invertref should be taken from %ARG instead of
from %MATCH.
Pseudo-subrules¶
Aliases can also be given to standard Perl subpatterns, as well as to code
blocks within a regex. The syntax for subpatterns is:
<ALIAS= (SUBPATTERN) >
In other words, the syntax is exactly like an aliased subrule call, except that
the rule name is replaced with a set of parentheses containing the subpattern.
Any parentheses--capturing or non-capturing--will do.
The effect of aliasing a standard subpattern is to cause whatever that
subpattern matches to be saved in the result-hash, using the alias as its key.
For example:
<rule: file_command>
<cmd=(mv|cp|ln)> <from=file> <to=file>
Here, the "<cmd=(mv|cp|ln)>" is treated exactly like a regular
"(mv|cp|ln)", but whatever substring it matches is saved in the
result-hash under the key 'cmd'.
The syntax for aliasing code blocks is:
<ALIAS= (?{ your($code->here) }) >
Note, however, that the code block must be specified in the standard Perl 5.10
regex notation: "(?{...})". A common mistake is to write:
<ALIAS= { your($code->here } >
instead, which will attempt to interpolate $code before the regex is even
compiled, as such variables are only "protected" from interpolation
inside a "(?{...})".
When correctly specified, this construct executes the code in the block and
saves the result of that execution in the result-hash, using the alias as its
key. Aliased code blocks are useful for adding semantic information based on
which branch of a rule is executed. For example, consider the
"copy_cmd" alternatives shown earlier:
<rule: copy_cmd>
copy <from=file> <to=file>
| dup <to=file> as <from=file>
| <from=file> -> <to=file>
| <to=file> <- <from=file>
Using aliased code blocks, you could add an extra field to the result- hash to
describe which form of the command was detected, like so:
<rule: copy_cmd>
copy <from=file> <to=file> <type=(?{ 'std' })>
| dup <to=file> as <from=file> <type=(?{ 'rev' })>
| <from=file> -> <to=file> <type=(?{ +1 })>
| <to=file> <- <from=file> <type=(?{ -1 })>
Now, if the rule matched, the result-hash would contain something like:
copy_cmd => {
from => 'oldfile',
to => 'newfile',
type => 'fwd',
}
Note that, in addition to the semantics described above, aliased subpatterns and
code blocks also become visible to Regexp::Grammars' integrated debugger (see
Debugging).
Aliased literals¶
As the previous example illustrates, it is inconveniently verbose to assign
constants via aliased code blocks. So Regexp::Grammars provides a short-cut.
It is possible to directly alias a numeric literal or a single-quote delimited
literal string, without putting either inside a code block. For example, the
previous example could also be written:
<rule: copy_cmd>
copy <from=file> <to=file> <type='std'>
| dup <to=file> as <from=file> <type='rev'>
| <from=file> -> <to=file> <type= +1 >
| <to=file> <- <from=file> <type= -1 >
Note that only these two forms of literal are supported in this abbreviated
syntax.
Amnesiac subrule calls¶
By default, every subrule call saves its result into the result-hash, either
under its own name, or under an alias.
However, sometimes you may want to refactor some literal part of a rule into one
or more subrules, without having those submatches added to the result-hash.
The syntax for calling a subrule, but ignoring its return value is:
<.SUBRULE>
(which is stolen directly from Perl 6).
For example, you may prefer to rewrite a rule such as:
<rule: paren_pair>
\(
(?: <escape> | <paren_pair> | <brace_pair> | [^()] )*
\)
without any literal matching, like so:
<rule: paren_pair>
<.left_paren>
(?: <escape> | <paren_pair> | <brace_pair> | <.non_paren> )*
<.right_paren>
<token: left_paren> \(
<token: right_paren> \)
<token: non_paren> [^()]
Moreover, as the individual components inside the parentheses probably aren't
being captured for any useful purpose either, you could further optimize that
to:
<rule: paren_pair>
<.left_paren>
(?: <.escape> | <.paren_pair> | <.brace_pair> | <.non_paren> )*
<.right_paren>
Note that you can also use the dot modifier on an aliased subpattern:
<.Alias= (SUBPATTERN) >
This seemingly contradictory behaviour (of giving a subpattern a name, then
deliberately ignoring that name) actually does make sense in one situation.
Providing the alias makes the subpattern visible to the debugger, while using
the dot stops it from affecting the result-hash. See "Debugging
non-grammars" for an example of this usage.
Private subrule calls¶
If a rule name (or an alias) begins with an underscore:
<_RULENAME> <_ALIAS=RULENAME>
<[_RULENAME]> <[_ALIAS=RULENAME]>
then matching proceeds as normal, and any result that is returned is stored in
the current result-hash in the usual way.
However, when any rule finishes (and just before it returns) it first filters
its result-hash, removing any entries whose keys begin with an underscore.
This means that any subrule with an underscored name (or with an underscored
alias) remembers its result, but only until the end of the current rule. Its
results are effectively private to the current rule.
This is especially useful in conjunction with result distillation.
Lookahead (zero-width) subrules¶
Non-capturing subrule calls can be used in normal lookaheads:
<rule: qualified_typename>
# A valid typename and has a :: in it...
(?= <.typename> ) [^\s:]+ :: \S+
<rule: identifier>
# An alpha followed by alnums (but not a valid typename)...
(?! <.typename> ) [^\W\d]\w*
but the syntax is a little unwieldy. More importantly, an internal problem with
backtracking causes positive lookaheads to mess up the module's named
capturing mechanism.
So Regexp::Grammars provides two shorthands:
<!typename> same as: (?! <.typename> )
<?typename> same as: (?= <.typename> ) ...but works correctly!
These two constructs can also be called with arguments, if necessary:
<rule: Command>
<Keyword>
(?:
<!Terminator(:Keyword)> <Args=(\S+)>
)?
<Terminator(:Keyword)>
Note that, as the above equivalences imply, neither of these forms of a
subroutine call ever captures what it matches.
Matching separated lists¶
One of the commonest tasks in text parsing is to match a list of unspecified
length, in which items are separated by a fixed token. Things like:
1, 2, 3 , 4 ,13, 91 # Numbers separated by commas and spaces
g-c-a-g-t-t-a-c-a # DNA bases separated by dashes
/usr/local/bin # Names separated by directory markers
/usr:/usr/local:bin # Directories separated by colons
The usual construct required to parse these kinds of structures is either:
<rule: list>
<item> <separator> <list> # recursive definition
| <item> # base case
or, if you want to allow zero-or-more items instead of requiring one-or-more:
<rule: list_opt>
<list>? # entire list may be missing
<rule: list> # as before...
<item> <separator> <list> # recursive definition
| <item> # base case
Or, more efficiently, but less prettily:
<rule: list>
<[item]> (?: <separator> <[item]> )* # one-or-more
<rule: list_opt>
(?: <[item]> (?: <separator> <[item]> )* )? # zero-or-more
Because separated lists are such a common component of grammars,
Regexp::Grammars provides cleaner ways to specify them:
<rule: list>
<[item]>+ % <separator> # one-or-more
<rule: list_zom>
<[item]>* % <separator> # zero-or-more
Note that these are just regular repetition qualifiers (i.e. "+" and
"*") applied to a subriule ("<[item]>"), with a
"%" modifier after them to specify the required separator between
the repeated matches.
The number of repetitions matched is controlled both by the nature of the
qualifier ("+" vs "*") and by the subrule specified after
the "%". The qualified subrule will be repeatedly matched for as
long as its qualifier allows, provided that the second subrule also matches
between those repetitions.
For example, you can match a parenthesized sequence of one-or-more numbers
separated by commas, such as:
(1, 2, 3, 4, 13, 91) # Numbers separated by commas (and spaces)
with:
<rule: number_list>
\( <[number]>+ % <comma> \)
<token: number> \d+
<token: comma> ,
Note that any spaces round the commas will be ignored because
"<number_list>" is specified as a rule and the "+%"
specifier has spaces within and around it. To disallow spaces around the
commas, make sure there are no spaces in or around the "+%":
<rule: number_list_no_spaces>
\( <[number]>+%<comma> \)
(or else specify the rule as a token instead).
Because the "%" is a modifier applied to a qualifier, you can modify
any other repetition qualifier in the same way. For example:
<[item]>{2,4} % <sep> # two-to-four items, separated
<[item]>{7} % <sep> # exactly 7 items, separated
<[item]>{10,}? % <sep> # minimum of 10 or more items, separated
You can even do this:
<[item]>? % <sep> # one-or-zero items, (theoretically) separated
though the separator specification is, of course, meaningless in that case as it
will never be needed to separate a maximum of one item.
If a "%" appears anywhere else in a grammar (i.e.
not
immediately after a repetition qualifier), it is treated normally (i.e. as a
self-matching literal character):
<token: perl_hash>
% <ident> # match "%foo", "%bar", etc.
<token: perl_mod>
<expr> % <expr> # match "$n % 2", "($n+3) % ($n-1)", etc.
If you need to match a literal "%" immediately after a repetition,
either quote it:
<token: percentage>
\d{1,3} \% solution # match "7% solution", etc.
or refactor the "%" character:
<token: percentage>
\d{1,3} <percent_sign> solution # match "7% solution", etc.
<token: percent_sign>
%
Note that it's usually necessary to use the "<[...]>" form for
the repeated items being matched, so that all of them are saved in the result
hash. You can also save all the separators (if they're important) by
specifying them as a list-like subrule too:
\( <[number]>* % <[comma]> \) # save numbers *and* separators
The repeated item
must be specified as a subrule call of some kind (i.e.
in angles), but the separators may be specified either as a subrule or as a
raw bracketed pattern. For example:
<[number]>* % ( , | : ) # Numbers separated by commas or colons
<[number]>* % [,:] # Same, but more efficiently matched
The separator should always be specified within matched delimiters of some kind:
either matching "<...>" or matching "(...)" or
matching "[...]". Simple, non-bracketed separators will sometimes
also work:
<[number]>+ % ,
but not always:
<[number]>+ % ,\s+ # Oops! Separator is just: ,
This is because of the limited way in which the module internally parses
ordinary regex components (i.e. without full understanding of their implicit
precedence). As a consequence, consistently placing brackets around any
separator is a much safer approach:
<[number]>+ % (,\s+)
You can also use a simple pattern on the left of the "%" as the item
matcher, but in this case it
must always be aliased into a
list-collecting subrule, like so:
<[item=(\d+)]>* % [,]
Note that, for backwards compatibility with earlier versions of
Regexp::Grammars, the "+%" operator can also be written:
"**". However, there can be no space between the two asterisks of
this variant. That is:
<[item]> ** <sep> # same as <[item]>* % <sep>
<[item]>* * <sep> # error (two * qualifiers in a row)
Matching hash keys¶
In some situations a grammar may need a rule that matches dozens, hundreds, or
even thousands of one-word alternatives. For example, when matching command
names, or valid userids, or English words. In such cases it is often
impractical (and always inefficient) to list all the alternatives between
"|" alterators:
<rule: shell_cmd>
a2p | ac | apply | ar | automake | awk | ...
# ...and 400 lines later
... | zdiff | zgrep | zip | zmore | zsh
<rule: valid_word>
a | aa | aal | aalii | aam | aardvark | aardwolf | aba | ...
# ...and 40,000 lines later...
... | zymotize | zymotoxic | zymurgy | zythem | zythum
To simplify such cases, Regexp::Grammars provides a special construct that
allows you to specify all the alternatives as the keys of a normal hash. The
syntax for that construct is simply to put the hash name inside angle brackets
(with no space between the angles and the hash name).
Which means that the rules in the previous example could also be written:
<rule: shell_cmd>
<%cmds>
<rule: valid_word>
<%dict>
provided that the two hashes (%cmds and %dict) are visible in the scope where
the grammar is created.
Matching a hash key in this way is typically
significantly faster than
matching a large set of alternations. Specifically, it is
O(length of
longest potential key) ^ 2, instead of
O(number of keys).
Internally, the construct is converted to something equivalent to:
<rule: shell_cmd>
(<.hk>) <require: (?{ exists $cmds{$CAPTURE} })>
<rule: valid_word>
(<.hk>) <require: (?{ exists $dict{$CAPTURE} })>
The special "<hk>" rule is created automatically, and defaults
to "\S+", but you can also define it explicitly to handle other
kinds of keys. For example:
<rule: hk>
[^\n]+ # Key may be any number of chars on a single line
<rule: hk>
[ACGT]{10,} # Key is a base sequence of at least 10 pairs
Alternatively, you can specify a different key-matching pattern for each hash
you're matching, by placing the required pattern in braces immediately after
the hash name. For example:
<rule: client_name>
# Valid keys match <.hk> (default or explicitly specified)
<%clients>
<rule: shell_cmd>
# Valid keys contain only word chars, hyphen, slash, or dot...
<%cmds { [\w-/.]+ }>
<rule: valid_word>
# Valid keyss contain only alphas or internal hyphen or apostrophe...
<%dict{ (?i: (?:[a-z]+[-'])* [a-z]+ ) }>
<rule: DNA_sequence>
# Valid keys are base sequences of at least 10 pairs...
<%sequences{[ACGT]{10,}}>
This second approach to key-matching is preferred, because it localizes any
non-standard key-matching behaviour to each individual hash.
Rematching subrule results¶
Sometimes it is useful to be able to rematch a string that has previously been
matched by some earlier subrule. For example, consider a rule to match
shell-like control blocks:
<rule: control_block>
for <expr> <[command]>+ endfor
| while <expr> <[command]>+ endwhile
| if <expr> <[command]>+ endif
| with <expr> <[command]>+ endwith
This would be much tidier if we could factor out the command names (which are
the only differences between the four alternatives). The problem is that the
obvious solution:
<rule: control_block>
<keyword> <expr>
<[command]>+
end<keyword>
doesn't work, because it would also match an incorrect input like:
for 1..10
echo $n
ls subdir/$n
endif
We need some way to ensure that the "<keyword>" matched
immediately after "end" is the same "<keyword>" that
was initially matched.
That's not difficult, because the first "<keyword>" will have
captured what it matched into $MATCH{keyword}, so we could just write:
<rule: control_block>
<keyword> <expr>
<[command]>+
end(??{quotemeta $MATCH{keyword}})
This is such a useful technique, yet so ugly, scary, and prone to error, that
Regexp::Grammars provides a cleaner equivalent:
<rule: control_block>
<keyword> <expr>
<[command]>+
end<\_keyword>
A directive of the form "<\_
IDENTIFIER>" is known as a
"matchref" (an abbreviation of "%MATCH-supplied
backreference"). Matchrefs always attempt to match, as a literal, the
current value of $MATCH{
IDENTIFIER}.
By default, a matchref does not capture what it matches, but you can have it do
so by giving it an alias:
<token: delimited_string>
<ldelim=str_delim> .*? <rdelim=\_ldelim>
<token: str_delim> ["'`]
At first glance this doesn't seem very useful as, by definition, $MATCH{ldelim}
and $MATCH{rdelim} must necessarily always end up with identical values.
However, it can be useful if the rule also has other alternatives and you want
to create a consistent internal representation for those alternatives, like
so:
<token: delimited_string>
<ldelim=str_delim> .*? <rdelim=\_ldelim>
| <ldelim=( \[ ) .*? <rdelim=( \] )
| <ldelim=( \{ ) .*? <rdelim=( \} )
| <ldelim=( \( ) .*? <rdelim=( \) )
| <ldelim=( \< ) .*? <rdelim=( \> )
You can also force a matchref to save repeated matches as a nested array, in the
usual way:
<token: marked_text>
<marker> <text> <[endmarkers=\_marker]>+
Be careful though, as the following will not do as you may expect:
<[marker]>+ <text> <[endmarkers=\_marker]>+
because the value of $MATCH{marker} will be an array reference, which the
matchref will flatten and concatenate, then match the resulting string as a
literal, which will mean the previous example will match endmarkers that are
exact multiples of the complete start marker, rather than endmarkers that
consist of any number of repetitions of the individual start marker delimiter.
So:
""text here""
""text here""""
""text here""""""
but not:
""text here"""
""text here"""""
Uneven start and end markers such as these are extremely unusual, so this
problem rarely arises in practice.
Note: Prior to Regexp::Grammars version 1.020, the
syntax for matchrefs was
"<\IDENTIFIER>" instead of
"<\_IDENTIFIER>". This
created problems when the identifier started with any of
"l" , "u",
"L" , "U",
"Q" , or "E", so the syntax
has had to be altered in a backwards incompatible way. It will not be
altered again.
Rematching balanced delimiters¶
Consider the example in the previous section:
<token: delimited_string>
<ldelim=str_delim> .*? <rdelim=\_ldelim>
| <ldelim=( \[ ) .*? <rdelim=( \] )
| <ldelim=( \{ ) .*? <rdelim=( \} )
| <ldelim=( \( ) .*? <rdelim=( \) )
| <ldelim=( \< ) .*? <rdelim=( \> )
The repeated pattern of the last four alternatives is gauling, but we can't just
refactor those delimiters as well:
<token: delimited_string>
<ldelim=str_delim> .*? <rdelim=\_ldelim>
| <ldelim=bracket> .*? <rdelim=\_ldelim>
because that would incorrectly match:
{ delimited content here {
while failing to match:
{ delimited content here }
To refactor balanced delimiters like those, we need a second kind of matchref;
one that's a little smarter.
Or, preferably, a lot smarter...because there are many other kinds of balanced
delimiters, apart from single brackets. For example:
{{{ delimited content here }}}
/* delimited content here */
(* delimited content here *)
`` delimited content here ''
if delimited content here fi
The common characteristic of these delimiter pairs is that the closing delimiter
is the
inverse of the opening delimiter: the sequence of characters is
reversed and certain characters (mainly brackets, but also
single-quotes/backticks) are mirror-reflected.
Regexp::Grammars supports the parsing of such delimiters with a construct known
as an
invertref, which is specified using the "</
IDENT>" directive. An invertref acts very like a matchref,
except that it does not convert to:
(??{ quotemeta( $MATCH{I<IDENT>} ) })
but rather to:
(??{ quotemeta( inverse( $MATCH{I<IDENT> ))} })
With this directive available, the balanced delimiters of the previous example
can be refactored to:
<token: delimited_string>
<ldelim=str_delim> .*? <rdelim=\_ldelim>
| <ldelim=( [[{(<] ) .*? <rdelim=/ldelim>
Like matchrefs, invertrefs come in the usual range of flavours:
</ident> # Match the inverse of $MATCH{ident}
<ALIAS=/ident> # Match inverse and capture to $MATCH{ident}
<[ALIAS=/ident]> # Match inverse and push on @{$MATCH{ident}}
The character pairs that are reversed during mirroring are: "{" and
"}", "[" and "]", "(" and
")", "<" and ">", "X" and
"X", "`" and "'".
The following mnemonics may be useful in distinguishing inverserefs from
backrefs: a backref starts with a "\" (just like the standard Perl
regex backrefs "\1" and "\g{-2}" and
"\k<name>"), whereas an inverseref starts with a "/"
(like an HTML or XML closing tag). Or just remember that
"<\_IDENT>" is "match the same again", and if you
want "the same again, only mirrored" instead, just mirror the
"\" to get "</IDENT>".
Rematching parametric results and delimiters¶
The "<\
IDENTIFIER>" and
"</
IDENTIFIER>" mechanisms normally locate the literal
to be matched by looking in $MATCH{
IDENTIFIER}.
However, you can cause them to look in $ARG{
IDENTIFIER} instead, by
prefixing the identifier with a single ":". This is especially
useful when refactoring subrules. For example, instead of:
<rule: Command>
<Keyword> <CommandBody> end_ <\_Keyword>
<rule: Placeholder>
<Keyword> \.\.\. end_ <\_Keyword>
you could parameterize the Terminator rule, like so:
<rule: Command>
<Keyword> <CommandBody> <Terminator(:Keyword)>
<rule: Placeholder>
<Keyword> \.\.\. <Terminator(:Keyword)>
<token: Terminator>
end_ <\:Keyword>
Tracking and reporting match positions¶
Regexp::Grammars automatically predefines a special token that makes it easy to
track exactly where in its input a particular subrule matches. That token is:
"<matchpos>".
The "<matchpos>" token implements a zero-width match that never
fails. It always returns the current index within the string that the grammar
is matching.
So, for example you could have your "<delimited_text>" subrule
detect and report unterminated text like so:
<token: delimited_text>
qq? <delim> <text=(.*?)> </delim>
|
<matchpos> qq? <delim>
<error: (?{"Unterminated string starting at index $MATCH{matchpos}"})>
Matching "<matchpos>" in the second alternative causes
$MATCH{matchpos} to contain the position in the string at which the
"<matchpos>" subrule was matched (in this example: the start
of the unterminated text).
If you want the line number instead of the string index, use the predefined
"<matchline>" subrule instead:
<token: delimited_text>
qq? <delim> <text=(.*?)> </delim>
| <matchline> qq? <delim>
<error: (?{"Unterminated string starting at line $MATCH{matchline}"})>
Note that the line numbers returned by "<matchline>" start at 1
(not at zero, as with "<matchpos>").
The "<matchpos>" and "<matchline>" subrules are
just like any other subrules; you can alias them
("<started_at=matchpos>") or match them repeatedly ( "(?:
<[matchline]> <[item]> )++"), etc.
Autoactions¶
The module also supports event-based parsing. You can specify a grammar in the
usual way and then, for a particular parse, layer a collection of call-backs
(known as "autoactions") over the grammar to handle the data as it
is parsed.
Normally, a grammar rule returns the result hash it has accumulated (or whatever
else was aliased to "MATCH=" within the rule). However, you can
specify an autoaction object before the grammar is matched.
Once the autoaction object is specified, every time a rule succeeds during the
parse, its result is passed to the object via one of its methods; specifically
it is passed to the method whose name is the same as the rule's.
For example, suppose you had a grammar that recognizes simple algebraic
expressions:
my $expr_parser = do{
use Regexp::Grammars;
qr{
<Expr>
<rule: Expr> <[Operand=Mult]>+ % <[Op=(\+|\-)]>
<rule: Mult> <[Operand=Pow]>+ % <[Op=(\*|/|%)]>
<rule: Pow> <[Operand=Term]>+ % <Op=(\^)>
<rule: Term> <MATCH=Literal>
| \( <MATCH=Expr> \)
<token: Literal> <MATCH=( [+-]? \d++ (?: \. \d++ )?+ )>
}xms
};
You could convert this grammar to a calculator, by installing a set of
autoactions that convert each rule's result hash to the corresponding value of
the sub-expression that the rule just parsed. To do that, you would create a
class with methods whose names match the rules whose results you want to
change. For example:
package Calculator;
use List::Util qw< reduce >;
sub new {
my ($class) = @_;
return bless {}, $class
}
sub Answer {
my ($self, $result_hash) = @_;
my $sum = shift @{$result_hash->{Operand}};
for my $term (@{$result_hash->{Operand}}) {
my $op = shift @{$result_hash->{Op}};
if ($op eq '+') { $sum += $term; }
else { $sum -= $term; }
}
return $sum;
}
sub Mult {
my ($self, $result_hash) = @_;
return reduce { eval($a . shift(@{$result_hash->{Op}}) . $b) }
@{$result_hash->{Operand}};
}
sub Pow {
my ($self, $result_hash) = @_;
return reduce { $b ** $a } reverse @{$result_hash->{Operand}};
}
Objects of this class (and indeed the class itself) now have methods
corresponding to some of the rules in the expression grammar. To apply those
methods to the results of the rules (as they parse) you simply install an
object as the "autoaction" handler, immediately before you initiate
the parse:
if ($text ~= $expr_parser->with_actions(Calculator->new)) {
say $/{Answer}; # Now prints the result of the expression
}
The "with_actions()" method expects to be passed an object or
classname. This object or class will be installed as the autoaction handler
for the next match against any grammar. After that match, the handler will be
uninstalled. "with_actions()" returns the grammar it's called on,
making it easy to call it as part of a match (which is the recommended idiom).
With a "Calculator" object set as the autoaction handler, whenever the
"Answer", "Mult", or "Pow" rule of the grammar
matches, the corresponding "Answer", "Mult", or
"Pow" method of the "Calculator" object will be called
(with the rule's result value passed as its only argument), and the result of
the method will be used as the result of the rule.
Note that nothing new happens when a "Term" or "Literal"
rule matches, because the "Calculator" object doesn't have methods
with those names.
The overall effect, then, is to allow you to specify a grammar without
rule-specific bahaviours and then, later, specify a set of final actions (as
methods) for some or all of the rules of the grammar.
Note that, if a particular callback method returns "undef", the result
of the corresponding rule will be passed through without modification.
Named grammars¶
All the grammars shown so far are confined to a single regex. However,
Regexp::Grammars also provides a mechanism that allows you to defined named
grammars, which can then be imported into other regexes. This gives the a way
of modularizing common grammatical components.
Defining a named grammar¶
You can create a named grammar using the "<grammar:...>"
directive. This directive must appear before the first rule definition in the
grammar, and instead of any start-rule. For example:
qr{
<grammar: List::Generic>
<rule: List>
<MATCH=[Item]>+ % <Separator>
<rule: Item>
\S++
<token: Separator>
\s* , \s*
}x;
This creates a grammar named "List::Generic", and installs it in the
module's internal caches, for future reference.
Note that there is no need (or reason) to assign the resulting regex to a
variable, as the named grammar cannot itself be matched against.
Using a named grammar¶
To make use of a named grammar, you need to incorporate it into another grammar,
by inheritance. To do that, use the "<extends:...>" directive,
like so:
my $parser = qr{
<extends: List::Generic>
<List>
}x;
The "<extends:...>" directive incorporates the rules defined in
the specified grammar into the current regex. You can then call any of those
rules in the start-pattern.
Overriding an inherited rule or token¶
Subrule dispatch within a grammar is always polymorphic. That is, when a subrule
is called, the most-derived rule of the same name within the grammar's
hierarchy is invoked.
So, to replace a particular rule within grammar, you simply need to inherit that
grammar and specify new, more-specific versions of any rules you want to
change. For example:
my $list_of_integers = qr{
<List>
# Inherit rules from base grammar...
<extends: List::Generic>
# Replace Item rule from List::Generic...
<rule: Item>
[+-]? \d++
}x;
You can also use "<extends:...>" in other named grammars, to
create hierarchies:
qr{
<grammar: List::Integral>
<extends: List::Generic>
<token: Item>
[+-]? <MATCH=(<.Digit>+)>
<token: Digit>
\d
}x;
qr{
<grammar: List::ColonSeparated>
<extends: List::Generic>
<token: Separator>
\s* : \s*
}x;
qr{
<grammar: List::Integral::ColonSeparated>
<extends: List::Integral>
<extends: List::ColonSeparated>
}x;
As shown in the previous example, Regexp::Grammars allows you to multiply
inherit two (or more) base grammars. For example, the
"List::Integral::ColonSeparated" grammar takes the definitions of
"List" and "Item" from the "List::Integral"
grammar, and the definition of "Separator" from
"List::ColonSeparated".
Note that grammars dispatch subrule calls using C3 method lookup, rather than
Perl's older DFS lookup. That's why "List::Integral::ColonSeparated"
correctly gets the more-specific "Separator" rule defined in
"List::ColonSeparated", rather than the more-generic version defined
in "List::Generic" (via "List::Integral"). See
"perldoc mro" for more discussion of the C3 dispatch algorithm.
Augmenting an inherited rule or token¶
Instead of replacing an inherited rule, you can augment it.
For example, if you need a grammar for lists of hexademical numbers, you could
inherit the behaviour of "List::Integral" and add the hex digits to
its "Digit" token:
my $list_of_hexadecimal = qr{
<List>
<extends: List::Integral>
<token: Digit>
<List::Integral::Digit>
| [A-Fa-f]
}x;
If you call a subrule using a fully qualified name (such as
"<List::Integral::Digit>"), the grammar calls that version of
the rule, rather than the most-derived version.
Debugging named grammars¶
Named grammars are independent of each other, even when inherited. This means
that, if debugging is enabled in a derived grammar, it will not be active in
any rules inherited from a base grammar, unless the base grammar also included
a "<debug:...>" directive.
This is a deliberate design decision, as activating the debugger adds a
significant amount of code to each grammar's implementation, which is
detrimental to the matching performance of the resulting regexes.
If you need to debug a named grammar, the best approach is to include a
"<debug: same>" directive at the start of the grammar. The
presence of this directive will ensure the necessary extra debugging code is
included in the regex implementing the grammar, while setting "same"
mode will ensure that the debugging mode isn't altered when the matcher uses
the inherited rules.
Common parsing techniques¶
Result distillation¶
Normally, calls to subrules produce nested result-hashes within the current
result-hash. Those nested hashes always have at least one automatically
supplied key (""), whose value is the entire substring that the
subrule matched.
If there are no other nested captures within the subrule, there will be no other
keys in the result-hash. This would be annoying as a typical nested grammar
would then produce results consisting of hashes of hashes, with each nested
hash having only a single key (""). This in turn would make
postprocessing the result-hash (in "%/") far more complicated than
it needs to be.
To avoid this behaviour, if a subrule's result-hash doesn't contain any keys
except "", the module "flattens" the result-hash, by
replacing it with the value of its single key.
So, for example, the grammar:
mv \s* <from> \s* <to>
<rule: from> [\w/.-]+
<rule: to> [\w/.-]+
doesn't return a result-hash like this:
{
"" => 'mv /usr/local/lib/libhuh.dylib /dev/null/badlib',
'from' => { "" => '/usr/local/lib/libhuh.dylib' },
'to' => { "" => '/dev/null/badlib' },
}
Instead, it returns:
{
"" => 'mv /usr/local/lib/libhuh.dylib /dev/null/badlib',
'from' => '/usr/local/lib/libhuh.dylib',
'to' => '/dev/null/badlib',
}
That is, because the 'from' and 'to' subhashes each have only a single entry,
they are each "flattened" to the value of that entry.
This flattening also occurs if a result-hash contains only "private"
keys (i.e. keys starting with underscores). For example:
mv \s* <from> \s* <to>
<rule: from> <_dir=path>? <_file=filename>
<rule: to> <_dir=path>? <_file=filename>
<token: path> [\w/.-]*/
<token: filename> [\w.-]+
Here, the "from" rule produces a result like this:
from => {
"" => '/usr/local/bin/perl',
_dir => '/usr/local/bin/',
_file => 'perl',
}
which is automatically stripped of "private" keys, leaving:
from => {
"" => '/usr/local/bin/perl',
}
which is then automatically flattened to:
from => '/usr/local/bin/perl'
List result distillation
A special case of result distillation occurs in a separated list, such as:
<rule: List>
<[Item]>+ % <[Sep=(,)]>
If this construct matches just a single item, the result hash will contain a
single entry consisting of a nested array with a single value, like so:
{ Item => [ 'data' ] }
Instead of returning this annoyingly nested data structure, you can tell
Regexp::Grammars to flatten it to just the inner data with a special
directive:
<rule: List>
<[Item]>+ % <[Sep=(,)]>
<minimize:>
The "<minimize:>" directive examines the result hash (i.e.
%MATCH). If that hash contains only a single entry, which is a reference to an
array with a single value, then the directive assigns that single value
directly to $MATCH, so that it will be returned instead of the usual result
hash.
This means that a normal separated list still results in a hash containing all
elements and separators, but a "degenerate" list of only one item
results in just that single item.
Manual result distillation
Regexp::Grammars also offers full manual control over the distillation process.
If you use the reserved word "MATCH" as the alias for a subrule
call:
<MATCH=filename>
or a subpattern match:
<MATCH=( \w+ )>
or a code block:
<MATCH=(?{ 42 })>
then the current rule will treat the return value of that subrule, pattern, or
code block as its complete result, and return that value instead of the usual
result-hash it constructs. This is the case even if the result has other
entries that would normally also be returned.
For example, in a rule like:
<rule: term>
<MATCH=literal>
| <left_paren> <MATCH=expr> <right_paren>
The use of "MATCH" aliases causes the rule to return either whatever
"<literal>" returns, or whatever "<expr>"
returns (provided it's between left and right parentheses).
Note that, in this second case, even though "<left_paren>" and
"<right_paren>"
are captured to the result-hash, they
are not returned, because the "MATCH" alias overrides the normal
"return the result-hash" semantics and returns only what its
associated subrule (i.e. "<expr>") produces.
Programmatic result distillation
It's also possible to control what a rule returns from within a code block.
Regexp::Grammars provides a set of reserved variables that give direct access
to the result-hash.
The result-hash itself can be accessed as %MATCH within any code block inside a
rule. For example:
<rule: sum>
<X=product> \+ <Y=product>
<MATCH=(?{ $MATCH{X} + $MATCH{Y} })>
Here, the rule matches a product (aliased 'X' in the result-hash), then a
literal '+', then another product (aliased to 'Y' in the result-hash). The
rule then executes the code block, which accesses the two saved values (as
$MATCH{X} and $MATCH{Y}), adding them together. Because the block is itself
aliased to "MATCH", the sum produced by the block becomes the (only)
result of the rule.
It is also possible to set the rule result from within a code block (instead of
aliasing it). The special "override" return value is represented by
the special variable $MATCH. So the previous example could be rewritten:
<rule: sum>
<X=product> \+ <Y=product>
(?{ $MATCH = $MATCH{X} + $MATCH{Y} })
Both forms are identical in effect. Any assignment to $MATCH overrides the
normal "return all subrule results" behaviour.
Assigning to $MATCH directly is particularly handy if the result may not always
be "distillable", for example:
<rule: sum>
<X=product> \+ <Y=product>
(?{ if (!ref $MATCH{X} && !ref $MATCH{Y}) {
# Reduce to sum, if both terms are simple scalars...
$MATCH = $MATCH{X} + $MATCH{Y};
}
else {
# Return full syntax tree for non-simple case...
$MATCH{op} = '+';
}
})
Note that you can also partially override the subrule return behaviour.
Normally, the subrule returns the complete text it matched as its context
substring (i.e. under the "empty key") in its result-hash. That is,
of course, $MATCH{""}, so you can override just that behaviour by
directly assigning to that entry.
For example, if you have a rule that matches key/value pairs from a
configuration file, you might prefer that any trailing comments not be
included in the "matched text" entry of the rule's result-hash. You
could hide such comments like so:
<rule: config_line>
<key> : <value> <comment>?
(?{
# Edit trailing comments out of "matched text" entry...
$MATCH = "$MATCH{key} : $MATCH{value}";
})
Some more examples of the uses of $MATCH:
<rule: FuncDecl>
# Keyword Name Keep return the name (as a string)...
func <Identifier> ; (?{ $MATCH = $MATCH{'Identifier'} })
<rule: NumList>
# Numbers in square brackets...
\[
( \d+ (?: , \d+)* )
\]
# Return only the numbers...
(?{ $MATCH = $CAPTURE })
<token: Cmd>
# Match standard variants then standardize the keyword...
(?: mv | move | rename ) (?{ $MATCH = 'mv'; })
Parse-time data processing¶
Using code blocks in rules, it's often possible to fully process data as you
parse it. For example, the "<sum>" rule shown in the previous
section might be part of a simple calculator, implemented entirely in a single
grammar. Such a calculator might look like this:
my $calculator = do{
use Regexp::Grammars;
qr{
<Answer>
<rule: Answer>
( <.Mult>+ % <.Op=([+-])> )
<MATCH= (?{ eval $CAPTURE })>
<rule: Mult>
( <.Pow>+ % <.Op=([*/%])> )
<MATCH= (?{ eval $CAPTURE })>
<rule: Pow>
<X=Term> \^ <Y=Pow>
<MATCH= (?{ $MATCH{X} ** $MATCH{Y}; })>
|
<MATCH=Term>
<rule: Term>
<MATCH=Literal>
| \( <MATCH=Answer> \)
<token: Literal>
<MATCH= ( [+-]? \d++ (?: \. \d++ )?+ )>
}xms
};
while (my $input = <>) {
if ($input =~ $calculator) {
say "--> $/{Answer}";
}
}
Because every rule computes a value using the results of the subrules below it,
and aliases that result to its "MATCH", each rule returns a complete
evaluation of the subexpression it matches, passing that back to higher-level
rules, which then do the same.
Hence, the result returned to the very top-level rule (i.e. to
"<Answer>") is the complete evaluation of the entire
expression that was matched. That means that, in the very process of having
matched a valid expression, the calculator has also computed the value of that
expression, which can then simply be printed directly.
It is often possible to have a grammar fully (or sometimes at least partially)
evaluate or transform the data it is parsing, and this usually leads to very
efficient and easy-to-maintain implementations.
The main limitation of this technique is that the data has to be in a
well-structured form, where subsets of the data can be evaluated using only
local information. In cases where the meaning of the data is distributed
through that data non-hierarchically, or relies on global state, or on
external information, it is often better to have the grammar simply construct
a complete syntax tree for the data first, and then evaluate that syntax tree
separately, after parsing is complete. The following section describes a
feature of Regexp::Grammars that can make this second style of data processing
simpler and more maintainable.
Object-oriented parsing¶
When a grammar has parsed successfully, the "%/" variable will contain
a series of nested hashes (and possibly arrays) representing the hierarchical
structure of the parsed data.
Typically, the next step is to walk that tree, extracting or converting or
otherwise processing that information. If the tree has nodes of many different
types, it can be difficult to build a recursive subroutine that can navigate
it easily.
A much cleaner solution is possible if the nodes of the tree are proper objects.
In that case, you just define a "process()" or
"traverse()" method for eah of the classes, and have every node call
that method on each of its children. For example, if the parser were to return
a tree of nodes representing the contents of a LaTeX file, then you could
define the following methods:
sub Latex::file::explain
{
my ($self, $level) = @_;
for my $element (@{$self->{element}}) {
$element->explain($level);
}
}
sub Latex::element::explain {
my ($self, $level) = @_;
( $self->{command} || $self->{literal})->explain($level)
}
sub Latex::command::explain {
my ($self, $level) = @_;
say "\t"x$level, "Command:";
say "\t"x($level+1), "Name: $self->{name}";
if ($self->{options}) {
say "\t"x$level, "\tOptions:";
$self->{options}->explain($level+2)
}
for my $arg (@{$self->{arg}}) {
say "\t"x$level, "\tArg:";
$arg->explain($level+2)
}
}
sub Latex::options::explain {
my ($self, $level) = @_;
$_->explain($level) foreach @{$self->{option}};
}
sub Latex::literal::explain {
my ($self, $level, $label) = @_;
$label //= 'Literal';
say "\t"x$level, "$label: ", $self->{q{}};
}
and then simply write:
if ($text =~ $LaTeX_parser) {
$/{LaTeX_file}->explain();
}
and the chain of "explain()" calls would cascade down the nodes of the
tree, each one invoking the appropriate "explain()" method according
to the type of node encountered.
The only problem is that, by default, Regexp::Grammars returns a tree of
plain-old hashes, not LaTeX::Whatever objects. Fortunately, it's easy to
request that the result hashes be automatically blessed into the appropriate
classes, using the "<objrule:...>" and
"<objtoken:...>" directives.
These directives are identical to the "<rule:...>" and
"<token:...>" directives (respectively), except that the rule
or token they create will also convert the hash it normally returns into an
object of a specified class. This conversion is done by passing the result
hash to the class's constructor:
$class->new(\%result_hash)
if the class has a constructor method named "new()", or else (if the
class doesn't provide a constructor) by directly blessing the result hash:
bless \%result_hash, $class
Note that, even if object is constructed via its own constructor, the module
still expects the new object to be hash-based, and will fail if the object is
anything but a blessed hash. The module issues an error in this case.
The generic syntax for these types of rules and tokens is:
<objrule: CLASS::NAME = RULENAME >
<objtoken: CLASS::NAME = TOKENNAME >
For example:
<objrule: LaTeX::Element=component>
# ...Defines a rule that can be called as <component>
# ...and which returns a hash-based LaTeX::Element object
<objtoken: LaTex::Literal=atom>
# ...Defines a token that can be called as <atom>
# ...and which returns a hash-based LaTeX::Literal object
Note that, just as in aliased subrule calls, the name by which something is
referred to outside the grammar (in this case, the class name) comes
before the "=", whereas the name that it is referred to
inside the grammar comes
after the "=".
You can freely mix object-returning and plain-old-hash-returning rules and
tokens within a single grammar, though you have to be careful not to
subsequently try to call a method on any of the unblessed nodes.
An important caveat regarding OO rules
Prior to Perl 5.14.0, Perl's regex engine was not fully re-entrant. This means
that in older versions of Perl, it is not possible to re-invoke the regex
engine when already inside the regex engine.
This means that you need to be careful that the "new()" constructors
that are called by your object-rules do not themselves use regexes in any way,
unless you're running under Perl 5.14 or later (in which case you can ignore
what follows).
The two ways this is most likely to happen are:
- 1.
- If you're using a class built on Moose, where one or more of the
"has" uses a type constraint (such as 'Int') that is implemented
via regex matching. For example:
has 'id' => (is => 'rw', isa => 'Int');
The workaround (for pre-5.14 Perls) is to replace the type constraint with
one that doesn't use a regex. For example:
has 'id' => (is => 'rw', isa => 'Num');
Alternatively, you could define your own type constraint that avoids
regexes:
use Moose::Util::TypeConstraints;
subtype 'Non::Regex::Int',
as 'Num',
where { int($_) == $_ };
no Moose::Util::TypeConstraints;
# and later...
has 'id' => (is => 'rw', isa => 'Non::Regex::Int');
- 2.
- If your class uses an "AUTOLOAD()" method to implement its
constructor and that method uses the typical:
$AUTOLOAD =~ s/.*://;
technique. The workaround here is to achieve the same effect without a
regex. For example:
my $last_colon_pos = rindex($AUTOLOAD, ':');
substr $AUTOLOAD, 0, $last_colon_pos+1, q{};
Note that this caveat against using nested regexes also applies to any code
blocks executed inside a rule or token (whether or not those rules or tokens
are object-oriented).
A naming shortcut
If an "<objrule:...>" or "<objtoken:...>" is
defined with a class name that is
not followed by "=" and a
rule name, then the rule name is determined automatically from the classname.
Specifically, the final component of the classname (i.e. after the last
"::", if any) is used.
For example:
<objrule: LaTeX::Element>
# ...Defines a rule that can be called as <Element>
# ...and which returns a hash-based LaTeX::Element object
<objtoken: LaTex::Literal>
# ...Defines a token that can be called as <Literal>
# ...and which returns a hash-based LaTeX::Literal object
<objtoken: Comment>
# ...Defines a token that can be called as <Comment>
# ...and which returns a hash-based Comment object
Debugging¶
Regexp::Grammars provides a number of features specifically designed to help
debug both grammars and the data they parse.
All debugging messages are written to a log file (which, by default, is just
STDERR). However, you can specify a disk file explicitly by placing a
"<logfile:...>" directive at the start of your grammar:
$grammar = qr{
<logfile: LaTeX_parser_log >
\A <LaTeX_file> \Z # Pattern to match
<rule: LaTeX_file>
# etc.
}x;
You can also explicitly specify that messages go to the terminal:
<logfile: - >
Debugging grammar creation with "<logfile:...>"¶
Whenever a log file has been directly specified, Regexp::Grammars automatically
does verbose static analysis of your grammar. That is, whenever it compiles a
grammar containing an explicit "<logfile:...>" directive it
logs a series of messages explaining how it has interpreted the various
components of that grammar. For example, the following grammar:
<logfile: parser_log >
<cmd>
<rule: cmd>
mv <from=file> <to=file>
| cp <source> <[file]> <.comment>?
would produce the following analysis in the 'parser_log' file:
info | Processing the main regex before any rule definitions
| |
| |...Treating <cmd> as:
| | | match the subrule <cmd>
| | \ saving the match in $MATCH{'cmd'}
| |
| \___End of main regex
|
info | Defining a rule: <cmd>
| |...Returns: a hash
| |
| |...Treating ' mv ' as:
| | \ normal Perl regex syntax
| |
| |...Treating <from=file> as:
| | | match the subrule <file>
| | \ saving the match in $MATCH{'from'}
| |
| |...Treating <to=file> as:
| | | match the subrule <file>
| | \ saving the match in $MATCH{'to'}
| |
| |...Treating ' | cp ' as:
| | \ normal Perl regex syntax
| |
| |...Treating <source> as:
| | | match the subrule <source>
| | \ saving the match in $MATCH{'source'}
| |
| |...Treating <[file]> as:
| | | match the subrule <file>
| | \ appending the match to $MATCH{'file'}
| |
| |...Treating <.comment>? as:
| | | match the subrule <comment> if possible
| | \ but don't save anything
| |
| \___End of rule definition
This kind of static analysis is a useful starting point in debugging a miscreant
grammar, because it enables you to see what you actually specified (as opposed
to what you
thought you'd specified).
Debugging grammar execution with "<debug:...>"¶
Regexp::Grammars also provides a simple interactive debugger, with which you can
observe the process of parsing and the data being collected in any
result-hash.
To initiate debugging, place a "<debug:...>" directive anywhere
in your grammar. When parsing reaches that directive the debugger will be
activated, and the command specified in the directive immediately executed.
The available commands are:
<debug: on> - Enable debugging, stop when a rule matches
<debug: match> - Enable debugging, stop when a rule matches
<debug: try> - Enable debugging, stop when a rule is tried
<debug: run> - Enable debugging, run until the match completes
<debug: same> - Continue debugging (or not) as currently
<debug: off> - Disable debugging and continue parsing silently
<debug: continue> - Synonym for <debug: run>
<debug: step> - Synonym for <debug: try>
These directives can be placed anywhere within a grammar and take effect when
that point is reached in the parsing. Hence, adding a
"<debug:step>" directive is very much like setting a
breakpoint at that point in the grammar. Indeed, a common debugging strategy
is to turn debugging on and off only around a suspect part of the grammar:
<rule: tricky> # This is where we think the problem is...
<debug:step>
<preamble> <text> <postscript>
<debug:off>
Once the debugger is active, it steps through the parse, reporting rules that
are tried, matches and failures, backtracking and restarts, and the parser's
location within both the grammar and the text being matched. That report looks
like this:
===============> Trying <grammar> from position 0
> cp file1 file2 |...Trying <cmd>
| |...Trying <cmd=(cp)>
| | \FAIL <cmd=(cp)>
| \FAIL <cmd>
\FAIL <grammar>
===============> Trying <grammar> from position 1
cp file1 file2 |...Trying <cmd>
| |...Trying <cmd=(cp)>
file1 file2 | | \_____<cmd=(cp)> matched 'cp'
file1 file2 | |...Trying <[file]>+
file2 | | \_____<[file]>+ matched 'file1'
| |...Trying <[file]>+
[eos] | | \_____<[file]>+ matched ' file2'
| |...Trying <[file]>+
| | \FAIL <[file]>+
| |...Trying <target>
| | |...Trying <file>
| | | \FAIL <file>
| | \FAIL <target>
<~~~~~~~~~~~~~~ | |...Backtracking 5 chars and trying new match
file2 | |...Trying <target>
| | |...Trying <file>
| | | \____ <file> matched 'file2'
[eos] | | \_____<target> matched 'file2'
| \_____<cmd> matched ' cp file1 file2'
\_____<grammar> matched ' cp file1 file2'
The first column indicates the point in the input at which the parser is trying
to match, as well as any backtracking or forward searching it may need to do.
The remainder of the columns track the parser's hierarchical traversal of the
grammar, indicating which rules are tried, which succeed, and what they match.
Provided the logfile is a terminal (as it is by default), the debugger also
pauses at various points in the parsing process--before trying a rule, after a
rule succeeds, or at the end of the parse--according to the most recent
command issued. When it pauses, you can issue a new command by entering a
single letter:
m - to continue until the next subrule matches
t or s - to continue until the next subrule is tried
r or c - to continue to the end of the grammar
o - to switch off debugging
Note that these are the first letters of the corresponding
"<debug:...>" commands, listed earlier. Just hitting ENTER
while the debugger is paused repeats the previous command.
While the debugger is paused you can also type a 'd', which will display the
result-hash for the current rule. This can be useful for detecting which rule
isn't returning the data you expected.
Resizing the context string
By default, the first column of the debugger output (which shows the current
matching position within the string) is limited to a width of 20 columns.
However, you can change that limit calling the
"Regexp::Grammars::set_context_width()" subroutine. You have to
specify the fully qualified name, however, as Regexp::Grammars does not export
this (or any other) subroutine.
"set_context_width()" expects a single argument: a positive integer
indicating the maximal allowable width for the context column. It issues a
warning if an invalid value is passed, and ignores it.
If called in a void context, "set_context_width()" changes the context
width permanently throughout your application. If called in a scalar or list
context, "set_context_width()" returns an object whose destructor
will cause the context width to revert to its previous value. This means you
can temporarily change the context width within a given block with something
like:
{
my $temporary = Regexp::Grammars::set_context_width(50);
if ($text =~ $parser) {
do_stuff_with( %/ );
}
} # <--- context width automagically reverts at this point
and the context width will change back to its previous value when $temporary
goes out of scope at the end of the block.
User-defined logging with "<log:...>"¶
Both static and interactive debugging send a series of predefined log messages
to whatever log file you have specified. It is also possible to send
additional, user-defined messages to the log, using the
"<log:...>" directive.
This directive expects either a simple text or a codeblock as its single
argument. If the argument is a code block, that code is expected to return the
text of the message; if the argument is anything else, that something else
is the literal message. For example:
<rule: ListElem>
<Elem= ( [a-z]\d+) >
<log: Checking for a suffix, too...>
<Suffix= ( : \d+ ) >?
<log: (?{ "ListElem: $MATCH{Elem} and $MATCH{Suffix}" })>
User-defined log messages implemented using a codeblock can also specify a
severity level. If the codeblock of a "<log:...>" directive
returns two or more values, the first is treated as a log message severity
indicator, and the remaining values as separate lines of text to be logged.
For example:
<rule: ListElem>
<Elem= ( [a-z]\d+) >
<Suffix= ( : \d+ ) >?
<log: (?{
warn => "Elem was: $MATCH{Elem}",
"Suffix was $MATCH{Suffix}",
})>
When they are encountered, user-defined log messages are interspersed between
any automatic log messages (i.e. from the debugger), at the correct level of
nesting for the current rule.
Debugging non-grammars¶
[Note that, with the release in 2012 of the Regexp::Debugger module (on
CPAN) the techniques described below are unnecessary. If you need to
debug plain Perl regexes, use Regexp::Debugger instead.]
It is possible to use Regexp::Grammars without creating
any subrule
definitions, simply to debug a recalcitrant regex. For example, if the
following regex wasn't working as expected:
my $balanced_brackets = qr{
\( # left delim
(?:
\\ # escape or
| (?R) # recurse or
| . # whatever
)*
\) # right delim
}xms;
you could instrument it with aliased subpatterns and then debug it step-by-step,
using Regexp::Grammars:
use Regexp::Grammars;
my $balanced_brackets = qr{
<debug:step>
<.left_delim= ( \( )>
(?:
<.escape= ( \\ )>
| <.recurse= ( (?R) )>
| <.whatever=( . )>
)*
<.right_delim= ( \) )>
}xms;
while (<>) {
say 'matched' if /$balanced_brackets/;
}
Note the use of amnesiac aliased subpatterns to avoid needlessly building a
result-hash. Alternatively, you could use listifying aliases to preserve the
matching structure as an additional debugging aid:
use Regexp::Grammars;
my $balanced_brackets = qr{
<debug:step>
<[left_delim= ( \( )]>
(?:
<[escape= ( \\ )]>
| <[recurse= ( (?R) )]>
| <[whatever=( . )]>
)*
<[right_delim= ( \) )]>
}xms;
if ( '(a(bc)d)' =~ /$balanced_brackets/) {
use Data::Dumper 'Dumper';
warn Dumper \%/;
}
Handling errors when parsing¶
Assuming you have correctly debugged your grammar, the next source of problems
will probably be invalid input (especially if that input is being provided
interactively). So Regexp::Grammars also provides some support for detecting
when a parse is likely to fail...and informing the user why.
Requirements¶
The "<require:...>" directive is useful for testing conditions
that it's not easy (or even possible) to check within the syntax of the the
regex itself. For example:
<rule: IPV4_Octet_Decimal>
# Up three digits...
<MATCH= ( \d{1,3}+ )>
# ...but less than 256...
<require: (?{ $MATCH <= 255 })>
A require expects a regex codeblock as its argument and succeeds if the final
value of that codeblock is true. If the final value is false, the directive
fails and the rule starts backtracking.
Note, in this example that the digits are matched with " \d{1,3}+ ".
The trailing "+" prevents the "{1,3}" repetition from
backtracking to a smaller number of digits if the
"<require:...>" fails.
Handling failure¶
The module has limited support for error reporting from within a grammar, in the
form of the "<error:...>" and "<warning:...>"
directives and their shortcuts: "<...>",
"<!!!>", and "<???>"
Error messages
The "<error: MSG>" directive queues a
conditional error
message within "@!" and then fails to match (that is, it is
equivalent to a "(?!)" when matching). For example:
<rule: ListElem>
<SerialNumber>
| <ClientName>
| <error: (?{ $errcount++ . ': Missing list element' })>
So a common code pattern when using grammars that do this kind of error
detection is:
if ($text =~ $grammar) {
# Do something with the data collected in %/
}
else {
say {*STDERR} $_ for @!; # i.e. report all errors
}
Each error message is conditional in the sense that, if any surrounding rule
subsequently matches, the message is automatically removed from
"@!". This implies that you can queue up as many error messages as
you wish, but they will only remain in "@!" if the match ultimately
fails. Moreover, only those error messages originating from rules that
actually contributed to the eventual failure-to-match will remain in
"@!".
If a code block is specified as the argument, the error message is whatever
final value is produced when the block is executed. Note that this final value
does not have to be a string (though it does have to be a scalar).
<rule: ListElem>
<SerialNumber>
| <ClientName>
| <error: (?{
# Return a hash, with the error information...
{ errnum => $errcount++, msg => 'Missing list element' }
})>
If anything else is specified as the argument, it is treated as a literal error
string (and may not contain an unbalanced '<' or '>', nor any
interpolated variables).
However, if the literal error string begins with "Expected " or
"Expecting ", then the error string automatically has the following
"context suffix" appended:
, but found '$CONTEXT' instead
For example:
qr{ <Arithmetic_Expression> # ...Match arithmetic expression
| # Or else
<error: Expected a valid expression> # ...Report error, and fail
# Rule definitions here...
}xms;
On an invalid input this example might produce an error message like:
"Expected a valid expression, but found '(2+3]*7/' instead"
The value of the special $CONTEXT variable is found by looking ahead in the
string being matched against, to locate the next sequence of non-blank
characters after the current parsing position. This variable may also be
explicitly used within the "<error: (?{...})>" form of the
directive.
As a special case, if you omit the message entirely from the directive, it is
supplied automatically, derived from the name of the current rule. For
example, if the following rule were to fail to match:
<rule: Arithmetic_expression>
<Multiplicative_Expression>+ % ([+-])
| <error:>
the error message queued would be:
"Expected arithmetic expression, but found 'one plus two' instead"
Note however, that it is still essential to include the colon in the directive.
A common mistake is to write:
<rule: Arithmetic_expression>
<Multiplicative_Expression>+ % ([+-])
| <error>
which merely attempts to call "<rule: error>" if the first
alternative fails.
Warning messages
Sometimes, you want to detect problems, but not invalidate the entire parse as a
result. For those occasions, the module provides a "less stringent"
form of error reporting: the "<warning:...>" directive.
This directive is exactly the same as an "<error:...>" in every
respect except that it does not induce a failure to match at the point it
appears.
The directive is, therefore, useful for reporting
non-fatal problems in a
parse. For example:
qr{ \A # ...Match only at start of input
<ArithExpr> # ...Match a valid arithmetic expression
(?:
# Should be at end of input...
\s* \Z
|
# If not, report the fact but don't fail...
<warning: Expected end-of-input>
<warning: (?{ "Extra junk at index $INDEX: $CONTEXT" })>
)
# Rule definitions here...
}xms;
Note that, because they do not induce failure, two or more
"<warning:...>" directives can be "stacked" in
sequence, as in the previous example.
Stubbing
The module also provides three useful shortcuts, specifically to make it easy to
declare, but not define, rules and tokens.
The "<...>" and "<???>" directives are
equivalent to the directive:
<error: Cannot match RULENAME (not implemented)>
The "<???>" is equivalent to the directive:
<warning: Cannot match RULENAME (not implemented)>
For example, in the following grammar:
<grammar: List::Generic>
<rule: List>
<[Item]>+ % (\s*,\s*)
<rule: Item>
<...>
the "Item" rule is declared but not defined. That means the grammar
will compile correctly, (the "List" rule won't complain about a call
to a non-existent "Item"), but if the "Item" rule isn't
overridden in some derived grammar, a match-time error will occur when
"List" tries to match the "<...>" within
"Item".
Localizing the (semi-)automatic error messages
Error directives of any of the following forms:
<error: Expecting identifier>
<error: >
<...>
<!!!>
or their warning equivalents:
<warning: Expecting identifier>
<warning: >
<???>
each autogenerate part or all of the actual error message they produce. By
default, that autogenerated message is always produced in English.
However, the module provides a mechanism by which you can intercept
every
error or warning that is queued to "@!" via these directives...and
localize those messages.
To do this, you call "Regexp::Grammars::set_error_translator()" (with
the full qualification, since Regexp::Grammars does not export it...nor
anything else, for that matter).
The "set_error_translator()" subroutine expect as single argument,
which must be a reference to another subroutine. This subroutine is then
called whenever an error or warning message is queued to "@!".
The subroutine is passed three arguments:
- •
- the message string,
- •
- the name of the rule from which the error or warning was queued, and
- •
- the value of $CONTEXT when the error or warning was encountered
The subroutine is expected to return the final version of the message that is
actually to be appended to "@!". To accomplish this it may make use
of one of the many internationalization/localization modules available in
Perl, or it may do the conversion entirely by itself.
The first argument is always exactly what appeared as a message in the original
directive (regardless of whether that message is supposed to trigger
autogeneration, or is just a "regular" error message). That is:
Directive 1st argument
<error: Expecting identifier> "Expecting identifier"
<warning: That's not a moon!> "That's not a moon!"
<error: > ""
<warning: > ""
<...> ""
<!!!> ""
<???> ""
The second argument always contains the name of the rule in which the directive
was encountered. For example, when invoked from within "<rule:
Frinstance>" the following directives produce:
Directive 2nd argument
<error: Expecting identifier> "Frinstance"
<warning: That's not a moon!> "Frinstance"
<error: > "Frinstance"
<warning: > "Frinstance"
<...> "-Frinstance"
<!!!> "-Frinstance"
<???> "-Frinstance"
Note that the "unimplemented" markers pass the rule name with a
preceding '-'. This allows your translator to distinguish between
"empty" messages (which should then be generated automatically) and
the "unimplemented" markers (which should report that the rule is
not yet properly defined).
If you call "Regexp::Grammars::set_error_translator()" in a void
context, the error translator is permanently replaced (at least, until the
next call to "set_error_translator()").
However, if you call "Regexp::Grammars::set_error_translator()" in a
scalar or list context, it returns an object whose destructor will restore the
previous translator. This allows you to install a translator only within a
given scope, like so:
{
my $temporary
= Regexp::Grammars::set_error_translator(\&my_translator);
if ($text =~ $parser) {
do_stuff_with( %/ );
}
else {
report_errors_in( @! );
}
} # <--- error translator automagically reverts at this point
Warning: any error translation subroutine you install will be called
during the grammar's parsing phase (i.e. as the grammar's regex is matching).
You should therefore ensure that your translator does not itself use regular
expressions, as nested evaluations of regexes inside other regexes are
extremely problematical (i.e. almost always disastrous) in Perl.
Restricting how long a parse runs¶
Like the core Perl 5 regex engine on which they are built, the grammars
implemented by Regexp::Grammars are essentially top-down parsers. This means
that they may occasionally require an exponentially long time to parse a
particular input. This usually occurs if a particular grammar includes a lot
of recursion or nested backtracking, especially if the grammar is then matched
against a long string.
The judicious use of non-backtracking repetitions (i.e. "x*+" and
"x++") can significantly improve parsing performance in many such
cases. Likewise, carefully reordering any high-level alternatives (so as to
test simple common cases first) can substantially reduce parsing times.
However, some languages are just intrinsically slow to parse using top-down
techniques (or, at least, may have slow-to-parse corner cases).
To help cope with this constraint, Regexp::Grammars provides a mechanism by
which you can limit the total effort that a given grammar will expend in
attempting to match. The "<timeout:...>" directive allows you
to specify how long a grammar is allowed to continue trying to match before
giving up. It expects a single argument, which must be an unsigned integer,
and it treats this integer as the number of seconds to continue attempting to
match.
For example:
<timeout: 10> # Give up after 10 seconds
indicates that the grammar should keep attempting to match for another 10
seconds from the point where the directive is encountered during a parse. If
the complete grammar has not matched in that time, the entire match is
considered to have failed, the matching process is immediately terminated, and
a standard error message ('Internal error: Timed out after 10 seconds (as
requested)') is returned in "@!".
A "<timeout:...>" directive can be placed anywhere in a grammar,
but is most usually placed at the very start, so that the entire grammar is
governed by the specified time limit. The second most common alternative is to
place the timeout at the start of a particular subrule that is known to be
potentially very slow.
A common mistake is to put the timeout specification at the top level of the
grammar, but place it
after the actual subrule to be matched, like so:
my $grammar = qr{
<Text_Corpus> # Subrule to be matched
<timeout: 10> # Useless use of timeout
<rule: Text_Corpus>
# et cetera...
}xms;
Since the parser will only reach the "<timeout: 10>" directive
after it has completely matched "<Text_Corpus>", the
timeout is only initiated at the very end of the matching process and so does
not limit that process in any useful way.
Immediate timeouts
As you might expect, a "<timeout: 0>" directive tells the parser
to keep trying for only zero more seconds, and therefore will immediately
cause the entire surrounding grammar to fail (no matter how deeply within that
grammar the directive is encountered).
This can occasionally be exteremely useful. If you know that detecting a
particular datum means that the grammar will never match, no matter how many
other alternatives may subsequently be tried, you can short-circuit the parser
by injecting a "<timeout: 0>" immediately after the offending
datum is detected.
For example, if your grammar only accepts certain versions of the language being
parsed, you could write:
<rule: Valid_Language_Version>
vers = <%AcceptableVersions>
|
vers = <bad_version=(\S++)>
<warning: (?{ "Cannot parse language version $MATCH{bad_version}" })>
<timeout: 0>
In fact, this "<warning: MSG> <timeout: 0>" sequence is
sufficiently useful, sufficiently complex, and sufficiently easy to get wrong,
that Regexp::Grammars provides a handy shortcut for it: the
"<fatal:...>" directive. A "<fatal:...>" is
exactly equivalent to a "<warning:...>" followed by a
zero-timeout, so the previous example could also be written:
<rule: Valid_Language_Version>
vers = <%AcceptableVersions>
|
vers = <bad_version=(\S++)>
<fatal: (?{ "Cannot parse language version $MATCH{bad_version}" })>
Like "<error:...>" and "<warning:...>",
"<fatal:...>" also provides its own failure context in
$CONTEXT, so the previous example could be further simplified to:
<rule: Valid_Language_Version>
vers = <%AcceptableVersions>
|
vers = <fatal:(?{ "Cannot parse language version $CONTEXT" })>
Also like "<error:...>", "<fatal:...>" can
autogenerate an error message if none is provided, so the example could be
still further reduced to:
<rule: Valid_Language_Version>
vers = <%AcceptableVersions>
|
vers = <fatal:>
In this last case, however, the error message returned in "@!" would
no longer be:
Cannot parse language version 0.95
It would now be:
Expected valid language version, but found '0.95' instead
Scoping considerations¶
If you intend to use a grammar as part of a larger program that contains other
(non-grammatical) regexes, it is more efficient--and less error-prone--to
avoid having Regexp::Grammars process those regexes as well. So it's often a
good idea to declare your grammar in a "do" block, thereby
restricting the scope of the module's effects.
For example:
my $grammar = do {
use Regexp::Grammars;
qr{
<file>
<rule: file>
<prelude>
<data>
<postlude>
<rule: prelude>
# etc.
}x;
};
Because the effects of Regexp::Grammars are lexically scoped, any regexes
defined outside that "do" block will be unaffected by the module.
INTERFACE¶
Perl API¶
- "use Regexp::Grammars;"
- Causes all regexes in the current lexical scope to be compile-time
processed for grammar elements.
- "$str =~ $grammar"
- "$str =~ /$grammar/"
- Attempt to match the grammar against the string, building a nested data
structure from it.
- "%/"
- This hash is assigned the nested data structure created by any successful
match of a grammar regex.
- "@!"
- This array is assigned the queue of error messages created by any
unsuccessful match attempt of a grammar regex.
Grammar syntax¶
Directives
- "<rule: IDENTIFIER>"
- Define a rule whose name is specified by the supplied identifier.
Everything following the "<rule:...>" directive (up to the
next "<rule:...>" or "<token:...>"
directive) is treated as part of the rule being defined.
Any whitespace in the rule is replaced by a call to the
"<.ws>" subrule (which defaults to matching
"\s*", but may be explicitly redefined).
- "<token: IDENTIFIER>"
- Define a rule whose name is specified by the supplied identifier.
Everything following the "<token:...>" directive (up to the
next "<rule:...>" or "<token:...>"
directive) is treated as part of the rule being defined.
Any whitespace in the rule is ignored (under the "/x" modifier),
or explicitly matched (if "/x" is not used).
- "<objrule: IDENTIFIER>"
- "<objtoken: IDENTIFIER>"
- Identical to a "<rule: IDENTIFIER>" or "<token:
IDENTIFIER>" declaration, except that the rule or token will also
bless the hash it normally returns, converting it to an object of a class
whose name is the same as the rule or token itself.
- "<require: (?{ CODE }) >"
- The code block is executed and if its final value is true, matching
continues from the same position. If the block's final value is false, the
match fails at that point and starts backtracking.
- "<error: (?{ CODE }) >"
- "<error: LITERAL TEXT >"
- "<error: >"
- This directive queues a conditional error message within the global
special variable "@!" and then fails to match at that point
(that is, it is equivalent to a "(?!)" or "(*FAIL)"
when matching).
- "<fatal: (?{ CODE }) >"
- "<fatal: LITERAL TEXT >"
- "<fatal: >"
- This directive is exactly the same as an "<error:...>" in
every respect except that it immediately causes the entire surrounding
grammar to fail, and parsing to immediate cease.
- "<warning: (?{ CODE }) >"
- "<warning: LITERAL TEXT >"
- This directive is exactly the same as an "<error:...>" in
every respect except that it does not induce a failure to match at the
point it appears. That is, it is equivalent to a "(?=)"
["succeed and continue matching"], rather than a
"(?!)" ["fail and backtrack"].
- "<debug: COMMAND >"
- During the matching of grammar regexes send debugging and warning
information to the specified log file (see "<logfile:
LOGFILE>").
The available "COMMAND"'s are:
<debug: continue> ___ Debug until end of complete parse
<debug: run> _/
<debug: on> ___ Debug until next subrule match
<debug: match> _/
<debug: try> ___ Debug until next subrule call or match
<debug: step> _/
<debug: same> ___ Maintain current debugging mode
<debug: off> ___ No debugging
See also the $DEBUG special variable.
- "<logfile: LOGFILE>"
- "<logfile: - >"
- During the compilation of grammar regexes, send debugging and warning
information to the specified LOGFILE (or to *STDERR if "-" is
specified).
If the specified LOGFILE name contains a %t, it is replaced with a
(sortable) "YYYYMMDD.HHMMSS" timestamp. For example:
<logfile: test-run-%t >
executed at around 9.30pm on the 21st of March 2009, would generate a log
file named: "test-run-20090321.213056"
- "<log: (?{ CODE }) >"
- "<log: LITERAL TEXT >"
- Append a message to the log file. If the argument is a code block, that
code is expected to return the text of the message; if the argument is
anything else, that something else is the literal message.
If the block returns two or more values, the first is treated as a log
message severity indicator, and the remaining values as separate lines of
text to be logged.
- "<timeout: INT >"
- Restrict the match-time of the parse to the specified number of seconds.
Queues a error message and terminates the entire match process if the
parse does not complete within the nominated time limit.
Subrule calls
- "<IDENTIFIER>"
- Call the subrule whose name is IDENTIFIER.
If it matches successfully, save the hash it returns in the current scope's
result-hash, under the key 'IDENTIFIER'.
- "<IDENTIFIER_1=IDENTIFIER_2>"
- Call the subrule whose name is IDENTIFIER_1.
If it matches successfully, save the hash it returns in the current scope's
result-hash, under the key 'IDENTIFIER_2'.
In other words, the "IDENTIFIER_1=" prefix changes the key under
which the result of calling a subrule is stored.
- "<.IDENTIFIER>"
- Call the subrule whose name is IDENTIFIER. Don't save the hash it returns.
In other words, the "dot" prefix disables saving of subrule
results.
- "<IDENTIFIER= ( PATTERN )>"
- Match the subpattern PATTERN.
If it matches successfully, capture the substring it matched and save that
substring in the current scope's result-hash, under the key
'IDENTIFIER'.
- "<.IDENTIFIER= ( PATTERN )>"
- Match the subpattern PATTERN. Don't save the substring it matched.
- "<IDENTIFIER= %HASH>"
- Match a sequence of non-whitespace then verify that the sequence is a key
in the specified hash
If it matches successfully, capture the sequence it matched and save that
substring in the current scope's result-hash, under the key
'IDENTIFIER'.
- "<%HASH>"
- Match a key from the hash. Don't save the substring it matched.
- "<IDENTIFIER= (?{ CODE })>"
- Execute the specified CODE.
Save the result (of the final expression that the CODE evaluates) in the
current scope's result-hash, under the key 'IDENTIFIER'.
- "<[IDENTIFIER]>"
- Call the subrule whose name is IDENTIFIER.
If it matches successfully, append the hash it returns to a nested array
within the current scope's result-hash, under the key
<'IDENTIFIER'>.
- "<[IDENTIFIER_1=IDENTIFIER_2]>"
- Call the subrule whose name is IDENTIFIER_1.
If it matches successfully, append the hash it returns to a nested array
within the current scope's result-hash, under the key 'IDENTIFIER_2'.
- "<ANY_SUBRULE>+ % <ANY_OTHER_SUBRULE>"
- "<ANY_SUBRULE>* % <ANY_OTHER_SUBRULE>"
- "<ANY_SUBRULE>+ % (PATTERN)"
- "<ANY_SUBRULE>* % (PATTERN)"
- Repeatedly call the first subrule. Keep matching as long as the subrule
matches, provided successive matches are separated by matches of the
second subrule or the pattern.
In other words, match a list of ANY_SUBRULE's separated by
ANY_OTHER_SUBRULE's or PATTERN's.
Note that, if a pattern is used to specify the separator, it must be
specified in some kind of matched parentheses. These may be capturing
["(...)"], non-capturing ["(?:...)"], non-backtracking
["(?>...)"], or any other construct enclosed by an opening
and closing paren.
Special variables within grammar actions¶
- $CAPTURE
- $CONTEXT
- These are both aliases for the built-in read-only $^N variable, which
always contains the substring matched by the nearest preceding
"(...)" capture. $^N still works perfectly well, but these are
provided to improve the readability of code blocks and error messages
respectively.
- $INDEX
- This variable contains the index at which the next match will be attempted
within the string being parsed. It is most commonly used in
"<error:...>" or "<log:...>" directives:
<rule: ListElem>
<log: (?{ "Trying words at index $INDEX" })>
<MATCH=( \w++ )>
|
<log: (?{ "Trying digits at index $INDEX" })>
<MATCH=( \d++ )>
|
<error: (?{ "Missing ListElem near index $INDEX" })>
- %MATCH
- This variable contains all the saved results of any subrules called from
the current rule. In other words, subrule calls like:
<ListElem> <Separator= (,)>
stores their respective match results in $MATCH{'ListElem'} and
$MATCH{'Separator'}.
- $MATCH
- This variable is an alias for $MATCH{"="}. This is the %MATCH
entry for the special "override value". If this entry is
defined, its value overrides the usual "return \%MATCH"
semantics of a successful rule.
- %ARG
- This variable contains all the key/value pairs that were passed into a
particular subrule call.
<Keyword> <Command> <Terminator(:Keyword)>
the "Terminator" rule could get access to the text matched by
"<Keyword>" like so:
<token: Terminator>
end_ (??{ $ARG{'Keyword'} })
Note that to match against the calling subrules 'Keyword' value, it's
necessary to use either a deferred interpolation ("(??{...})")
or a qualified matchref:
<token: Terminator>
end_ <\:Keyword>
A common mistake is to attempt to directly interpolate the argument:
<token: Terminator>
end_ $ARG{'Keyword'}
This evaluates $ARG{'Keyword'} when the grammar is compiled, rather than
when the rule is matched.
- $_
- At the start of any code blocks inside any regex, the variable $_ contains
the complete string being matched against. The current matching position
within that string is given by: "pos($_)".
- $DEBUG
- This variable stores the current debugging mode (which may be any of:
'off', 'on', 'run', 'continue', 'match', 'step', or 'try'). It is set
automatically by the "<debug:...>" command, but may also
be set manually in a code block (which can be useful for conditional
debugging). For example:
<rule: ListElem>
<Identifier>
# Conditionally debug if 'foobar' encountered...
(?{ $DEBUG = $MATCH{Identifier} eq 'foobar' ? 'step' : 'off' })
<Modifier>?
See also: the "<log: LOGFILE>" and "<debug:
DEBUG_CMD>" directives.
IMPORTANT CONSTRAINTS AND LIMITATIONS¶
- •
- Prior to Perl 5.14, the Perl 5 regex engine as not reentrant. So any
attempt to perform a regex match inside a "(?{ ... })" or
"(??{ ... })" under Perl 5.12 or earlier will almost certainly
lead to either weird data corruption or a segfault.
The same calamities can also occur in any constructor called by
"<objrule:>". If the constructor invokes another regex in
any way, it will most likely fail catastrophically. In particular, this
means that Moose constructors will frequently crash and burn within a
Regex::Grammars grammar (for example, if the Moose-based class declares an
attribute type constraint such as 'Int', which Moose checks using a
regex).
- •
- The additional regex constructs this module provides are implemented by
rewriting regular expressions. This is a (safer) form of source filtering,
but still subject to all the same limitations and fallibilities of any
other macro-based solution.
- •
- In particular, rewriting the macros involves the insertion of (a lot of)
extra capturing parentheses. This means you can no longer assume that
particular capturing parens correspond to particular numeric variables:
i.e. to $1, $2, $3 etc. If you want to capture directly use Perl 5.10's
named capture construct:
(?<name> [^\W\d]\w* )
Better still, capture the data in its correct hierarchical context using the
module's "named subpattern" construct:
<name= ([^\W\d]\w*) >
- •
- No recursive descent parser--including those created with
Regexp::Grammars--can directly handle left-recursive grammars with rules
of the form:
<rule: List>
<List> , <ListElem>
If you find yourself attempting to write a left-recursive grammar (which
Perl 5.10 may or may not complain about, but will never successfully parse
with), then you probably need to use the "separated list"
construct instead:
<rule: List>
<[ListElem]>+ % (,)
- •
- Grammatical parsing with Regexp::Grammars can fail if your grammar places
"non-backtracking" directives (i.e. the "(?>...)"
block or the "?+", "*+", or "++" repetition
specifiers) around a subrule call. The problem appears to be that
preventing the regex from backtracking through the in-regex actions that
Regexp::Grammars adds causes the module's internal stack to fall out of
sync with the regex match.
For the time being, you need to make sure that grammar rules don't appear
inside a "non-backtracking" directive.
- •
- Similarly, parsing with Regexp::Grammars will fail if your grammar places
a subrule call within a positive look-ahead, since these don't play nicely
with the data stack.
This seems to be an internal problem with perl itself. Investigations, and
attempts at a workaround, are proceeding.
For the time being, you need to make sure that grammar rules don't appear
inside a positive lookahead or use the "<?RULENAME>"
construct instead
DIAGNOSTICS¶
Note that (because the author cannot find a way to throw exceptions from within
a regex) none of the following diagnostics actually throws an exception.
Instead, these messages are simply written to the specified parser logfile (or
to *STDERR, if no logfile is specified).
However, any fatal match-time message will immediately terminate the parser
matching and will still set $@ (as if an exception had been thrown and caught
at that point in the code). You then have the option to check $@ immediately
after matching with the grammar, and rethrow if necessary:
if ($input =~ $grammar) {
process_data_in(\%/);
}
else {
die if $@;
}
- "Found call to %s, but no %s was defined in the grammar"
- You specified a call to a subrule for which there was no definition in the
grammar. Typically that's either because you forget to define the rule, or
because you misspelled either the definition or the subrule call. For
example:
<file>
<rule: fiel> <---- misspelled rule
<lines> <---- used but never defined
Regexp::Grammars converts any such subrule call attempt to an instant
catastrophic failure of the entire parse, so if your parser ever actually
tries to perform that call, Very Bad Things will happen.
- "Entire parse terminated prematurely while attempting to call
non-existent rule: %s"
- You ignored the previous error and actually tried to call to a subrule for
which there was no definition in the grammar. Very Bad Things are now
happening. The parser got very upset, took its ball, and went home. See
the preceding diagnostic for remedies.
This diagnostic should throw an exception, but can't. So it sets $@ instead,
allowing you to trap the error manually if you wish.
- "Fatal error: <objrule: %s> returned a non-hash-based
object"
- An <objrule:> was specified and returned a blessed object that
wasn't a hash. This will break the behaviour of the grammar, so the module
immediately reports the problem and gives up.
The solution is to use only hash-based classes with <objrule:>
- "Can't match against <grammar: %s>"
- The regex you attempted to match against defined a pure grammar, using the
"<grammar:...>" directive. Pure grammars have no
start-pattern and hence cannot be matched against directly.
You need to define a matchable grammar that inherits from your pure grammar
and then calls one of its rules. For example, instead of:
my $greeting = qr{
<grammar: Greeting>
<rule: greet>
Hi there
| Hello
| Yo!
}xms;
you need:
qr{
<grammar: Greeting>
<rule: greet>
Hi there
| Hello
| Yo!
}xms;
my $greeting = qr{
<extends: Greeting>
<greet>
}xms;
- "Inheritance from unknown grammar requested by <%s>"
- You used an "<extends:...>" directive to request that your
grammar inherit from another, but the grammar you asked to inherit from
doesn't exist.
Check the spelling of the grammar name, and that it's already been defined
somewhere earlier in your program.
- "Redeclaration of <%s> will be ignored"
- You defined two or more rules or tokens with the same name. The first one
defined in the grammar will be used; the rest will be ignored.
To get rid of the warning, get rid of the extra definitions (or, at least,
comment them out or rename the rules).
- "Possible invalid subrule call %s"
- Your grammar contained something of the form:
<identifier
<.identifier
<[identifier
which you might have intended to be a subrule call, but which didn't
correctly parse as one. If it was supposed to be a Regexp::Grammars
subrule call, you need to check the syntax you used. If it wasn't supposed
to be a subrule call, you can silence the warning by rewriting it and
quoting the leading angle:
\<identifier
\<.identifier
\<[identifier
- "Possible failed attempt to specify a directive: %s"
- Your grammar contained something of the form:
<identifier:...
but which wasn't a known directive like "<rule:...>" or
"<debug:...>". If it was supposed to be a Regexp::Grammars
directive, check the spelling of the directive name. If it wasn't supposed
to be a directive, you can silence the warning by rewriting it and quoting
the leading angle:
\<identifier:
- "Possible failed attempt to specify a subrule call %s"
- Your grammar contained something of the form:
<identifier...
but which wasn't a call to a known subrule like "<ident>" or
"<name>". If it was supposed to be a Regexp::Grammars
subrule call, check the spelling of the rule name in the angles. If it
wasn't supposed to be a subrule call, you can silence the warning by
rewriting it and quoting the leading angle:
\<identifier...
- "Repeated subrule %s will only capture its final match"
- You specified a subrule call with a repetition qualifier, such as:
<ListElem>*
or:
<ListElem>+
Because each subrule call saves its result in a hash entry of the same name,
each repeated match will overwrite the previous ones, so only the last
match will ultimately be saved. If you want to save all the matches, you
need to tell Regexp::Grammars to save the sequence of results as a nested
array within the hash entry, like so:
<[ListElem]>*
or:
<[ListElem]>+
If you really did intend to throw away every result but the final one, you
can silence the warning by placing the subrule call inside any kind of
parentheses. For example:
(<ListElem>)*
or:
(?: <ListElem> )+
- "Unable to open log file '$filename' (%s)"
- You specified a "<logfile:...>" directive but the file
whose name you specified could not be opened for writing (for the reason
given in the parens).
Did you misspell the filename, or get the permissions wrong somewhere in the
filepath?
- "Non-backtracking subrule %s may not revert correctly during
backtracking"
- Because of inherent limitations in the Perl regex engine, non-backtracking
constructs like "++", "*+", "?+", and
"(?>...)" do not always work correctly when applied to
subrule calls, especially in earlier versions of Perl.
If the grammar doesn't work properly, replace the offending constructs with
regular backtracking versions instead. If the grammar does work, you can
silence the warning by enclosing the subrule call in any kind of
parentheses. For example, change:
<[ListElem]>++
to:
(?: <[ListElem]> )++
- "Unexpected item before first subrule specification in definition of
<grammar: %s>"
- Named grammar definitions must consist only of rule and token definitions.
They cannot have patterns before the first definitions. You had some kind
of pattern before the first definition, which will be completely ignored
within the grammar.
To silence the warning, either comment out or delete whatever is before the
first rule/token definition.
- "No main regex specified before rule definitions"
- You specified an unnamed grammar (i.e. no "<grammar:...>"
directive), but didn't specify anything for it to actually match, just
some rules that you don't actually call. For example:
my $grammar = qr{
<rule: list> \( <item> +% [,] \)
<token: item> <list> | \d+
}x;
You have to provide something before the first rule to start the matching
off. For example:
my $grammar = qr{
<list> # <--- This tells the grammar how to start matching
<rule: list> \( <item> +% [,] \)
<token: item> <list> | \d+
}x;
- "Ignoring useless empty <ws:> directive"
- The "<ws:...>" directive specifies what whitespace matches
within the current rule. An empty "<ws:>" directive would
cause whitespace to match nothing at all, which is what happens in a token
definition, not in a rule definition.
Either put some subpattern inside the empty "<ws:...>" or,
if you really do want whitespace to match nothing at all, remove the
directive completely and change the rule definition to a token
definition.
- "Ignoring useless <ws: %s > directive in a token
definition"
- The "<ws:...>" directive is used to specify what
whitespace matches within a rule. Since whitespace never matches anything
inside tokens, putting a "<ws:...>" directive in a token
is a waste of time.
Either remove the useless directive, or else change the surrounding token
definition to a rule definition.
- "Quantifier that doesn't quantify anything: <%s>"
- You specified a rule or token something like:
<token: star> *
or:
<rule: add_op> plus | add | +
but the "*" and "+" in those examples are both regex
meta-operators: quantifiers that usually cause what precedes them to match
repeatedly. In these cases however, nothing is preceding the quantifier,
so it's a Perl syntax error.
You almost certainly need to escape the meta-characters in some way. For
example:
<token: star> \*
<rule: add_op> plus | add | [+]
CONFIGURATION AND ENVIRONMENT¶
Regexp::Grammars requires no configuration files or environment variables.
DEPENDENCIES¶
This module only works under Perl 5.10 or later.
INCOMPATIBILITIES¶
This module is likely to be incompatible with any other module that
automagically rewrites regexes. For example it may conflict with
Regexp::DefaultFlags, Regexp::DeferredExecution, or Regexp::Extended.
BUGS¶
No bugs have been reported.
Please report any bugs or feature requests to
"bug-regexp-grammars@rt.cpan.org", or through the web interface at
<
http://rt.cpan.org>.
AUTHOR¶
Damian Conway "<DCONWAY@CPAN.org>"
LICENCE AND COPYRIGHT¶
Copyright (c) 2009, Damian Conway "<DCONWAY@CPAN.org>". All
rights reserved.
This module is free software; you can redistribute it and/or modify it under the
same terms as Perl itself. See perlartistic.
DISCLAIMER OF WARRANTY¶
BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE
SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE
STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE
SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR
IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO
THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH YOU. SHOULD THE SOFTWARE
PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR, OR
CORRECTION.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY
COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE
SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE SOFTWARE (INCLUDING BUT NOT LIMITED TO
LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR
THIRD PARTIES OR A FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER
SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.