table of contents
- NAME
- DESCRIPTION
- DATA TYPES
- EXPORTS
- PERL LIKE REGULAR EXPRESSIONS SYNTAX
- PCRE REGULAR EXPRESSION DETAILS
- SPECIAL START-OF-PATTERN ITEMS
- CHARACTERS AND METACHARACTERS
- BACKSLASH
- CIRCUMFLEX AND DOLLAR
- FULL STOP (PERIOD, DOT) AND \N
- MATCHING A SINGLE DATA UNIT
- SQUARE BRACKETS AND CHARACTER CLASSES
- POSIX CHARACTER CLASSES
- VERTICAL BAR
- INTERNAL OPTION SETTING
- SUBPATTERNS
- DUPLICATE SUBPATTERN NUMBERS
- NAMED SUBPATTERNS
- REPETITION
- ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
- BACK REFERENCES
- ASSERTIONS
- CONDITIONAL SUBPATTERNS
- COMMENTS
- RECURSIVE PATTERNS
- SUBPATTERNS AS SUBROUTINES
- ONIGURUMA SUBROUTINE SYNTAX
- BACKTRACKING CONTROL
other versions
- jessie 1:17.3-dfsg-4+deb8u2
- jessie-backports 1:19.2.1+dfsg-2+deb9u1~bpo8+1
- stretch 1:19.2.1+dfsg-2+deb9u2
- testing 1:21.2.6+dfsg-1
- unstable 1:21.2.6+dfsg-1
- experimental 1:22.0~rc1+dfsg-1
other sections
re(3erl) | Erlang Module Definition | re(3erl) |
NAME¶
re - Perl like regular expressions for ErlangDESCRIPTION¶
This module contains regular expression matching functions for strings and binaries. The regular expression syntax and semantics resemble that of Perl. The library's matching algorithms are currently based on the PCRE library, but not all of the PCRE library is interfaced and some parts of the library go beyond what PCRE offers. The sections of the PCRE documentation which are relevant to this module are included here.Note:
The Erlang literal syntax for strings uses the "\" (backslash)
character as an escape code. You need to escape backslashes in literal
strings, both in your code and in the shell, with an additional backslash,
i.e.: "\\".
DATA TYPES¶
mp() = {re_pattern, term(), term(), term(), term()}
Opaque datatype containing a compiled regular expression. The mp() is guaranteed
to be a tuple() having the atom 're_pattern' as its first element, to allow
for matching in guards. The arity of the tuple() or the content of the other
fields may change in future releases.
nl_spec() = cr | crlf | lf | anycrlf | anycompile_option() = unicode| anchored| caseless| dollar_endonly| dotall| extended| firstline| multiline| no_auto_capture| dupnames| ungreedy| {newline, nl_spec()}| bsr_anycrlf| bsr_unicode| no_start_optimize| ucp| never_utf
EXPORTS¶
compile(Regexp) -> {ok, MP} | {error, ErrSpec}
Types:
Regexp = iodata()
MP = mp()
ErrSpec =
{ErrString :: string(), Position :: integer() >= 0}
{ErrString :: string(), Position :: integer() >= 0}
The same as compile(Regexp,[])
compile(Regexp, Options) -> {ok, MP} | {error, ErrSpec}
Types:
Regexp = iodata() | unicode:charlist()
Options = [Option]
Option = compile_option()
MP = mp()
ErrSpec =
{ErrString :: string(), Position :: integer() >= 0}
{ErrString :: string(), Position :: integer() >= 0}
This function compiles a regular expression with the syntax described below into
an internal format to be used later as a parameter to the run/2,3 functions.
Compiling the regular expression before matching is useful if the same
expression is to be used in matching against multiple subjects during the
program's lifetime. Compiling once and executing many times is far more
efficient than compiling each time one wants to match.
When the unicode option is given, the regular expression should be given as a
valid Unicode charlist(), otherwise as any valid iodata().
The options have the following meanings:
- unicode:
- The regular expression is given as a Unicode charlist() and the resulting regular expression code is to be run against a valid Unicode charlist() subject. Also consider the ucp option when using Unicode characters.
- anchored:
- The pattern is forced to be "anchored", that is, it is constrained to match only at the first matching point in the string that is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself.
- caseless:
- Letters in the pattern match both upper and lower case letters. It is equivalent to Perl's /i option, and it can be changed within a pattern by a (?i) option setting. Uppercase and lowercase letters are defined as in the ISO-8859-1 character set.
- dollar_endonly:
- A dollar metacharacter in the pattern matches only at the end of the subject string. Without this option, a dollar also matches immediately before a newline at the end of the string (but not before any other newlines). The dollar_endonly option is ignored if multiline is given. There is no equivalent option in Perl, and no way to set it within a pattern.
- dotall:
- A dot in the pattern matches all characters, including those that indicate newline. Without it, a dot does not match when the current position is at a newline. This option is equivalent to Perl's /s option, and it can be changed within a pattern by a (?s) option setting. A negative class such as [^a] always matches newline characters, independent of this option's setting.
- extended:
- Whitespace data characters in the pattern are ignored except when escaped or inside a character class. Whitespace does not include the VT character (ASCII 11). In addition, characters between an unescaped # outside a character class and the next newline, inclusive, are also ignored. This is equivalent to Perl's /x option, and it can be changed within a pattern by a (?x) option setting. This option makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters may never appear within special character sequences in a pattern, for example within the sequence (?( which introduces a conditional subpattern.
- firstline:
- An unanchored pattern is required to match before or at the first newline in the subject string, though the matched text may continue over the newline.
- multiline:
- By default, PCRE treats the subject string as consisting of a single line of characters (even if it actually contains newlines). The "start of line" metacharacter (^) matches only at the start of the string, while the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (unless dollar_endonly is given). This is the same as Perl.
When multiline is given, the "start of line" and "end of
line" constructs match immediately following or immediately before
internal newlines in the subject string, respectively, as well as at the very
start and end. This is equivalent to Perl's /m option, and it can be changed
within a pattern by a (?m) option setting. If there are no newlines in a
subject string, or no occurrences of ^ or $ in a pattern, setting
multiline has no effect.
- no_auto_capture:
- Disables the use of numbered capturing parentheses in the pattern. Any opening parenthesis that is not followed by ? behaves as if it were followed by ?: but named parentheses can still be used for capturing (and they acquire numbers in the usual way). There is no equivalent of this option in Perl.
- dupnames:
- Names used to identify capturing subpatterns need not be unique. This can be helpful for certain types of pattern when it is known that only one instance of the named subpattern can ever be matched. There are more details of named subpatterns below
- ungreedy:
- This option inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by "?". It is not compatible with Perl. It can also be set by a (?U) option setting within the pattern.
- {newline, NLSpec}:
- Override the default definition of a newline in the subject string, which is LF (ASCII 10) in Erlang.
- cr:
- Newline is indicated by a single character CR (ASCII 13)
- lf:
- Newline is indicated by a single character LF (ASCII 10), the default
- crlf:
- Newline is indicated by the two-character CRLF (ASCII 13 followed by ASCII 10) sequence.
- anycrlf:
- Any of the three preceding sequences should be recognized.
- any:
- Any of the newline sequences above, plus the Unicode sequences VT (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029).
- bsr_anycrlf:
- Specifies specifically that \R is to match only the cr, lf or crlf sequences, not the Unicode specific newline characters.
- bsr_unicode:
- Specifies specifically that \R is to match all the Unicode newline characters (including crlf etc, the default).
- no_start_optimize:
- This option disables optimization that may malfunction if "Special start-of-pattern items" are present in the regular expression. A typical example would be when matching "DEFABC" against "(*COMMIT)ABC", where the start optimization of PCRE would skip the subject up to the "A" and would never realize that the (*COMMIT) instruction should have made the matching fail. This option is only relevant if you use "start-of-pattern items", as discussed in the section "PCRE regular expression details" below.
- ucp:
- Specifies that Unicode Character Properties should be used when resolving \B, \b, \D, \d, \S, \s, \Wand \w. Without this flag, only ISO-Latin-1 properties are used. Using Unicode properties hurts performance, but is semantically correct when working with Unicode characters beyond the ISO-Latin-1 range.
- never_utf:
- Specifies that the (*UTF) and/or (*UTF8) "start-of-pattern items" are forbidden. This flag can not be combined with unicode. Useful if ISO-Latin-1 patterns from an external source are to be compiled.
inspect(MP, Item) -> {namelist, [binary()]}
Types:
MP = mp()
Item = namelist
This function takes a compiled regular expression and an item, returning the
relevant data from the regular expression. Currently the only supported item
is namelist, which returns the tuple {namelist, [ binary()]},
containing the names of all (unique) named subpatterns in the regular
expression.
Example:
1> {ok,MP} = re:compile("(?<A>A)|(?<B>B)|(?<C>C)"). {ok,{re_pattern,3,0,0, <<69,82,67,80,119,0,0,0,0,0,0,0,1,0,0,0,255,255,255,255, 255,255,...>>}} 2> re:inspect(MP,namelist). {namelist,[<<"A">>,<<"B">>,<<"C">>]} 3> {ok,MPD} = re:compile("(?<C>A)|(?<B>B)|(?<C>C)",[dupnames]). {ok,{re_pattern,3,0,0, <<69,82,67,80,119,0,0,0,0,0,8,0,1,0,0,0,255,255,255,255, 255,255,...>>}} 4> re:inspect(MPD,namelist). {namelist,[<<"B">>,<<"C">>]}Note specifically in the second example that the duplicate name only occurs once in the returned list, and that the list is in alphabetical order regardless of where the names are positioned in the regular expression. The order of the names is the same as the order of captured subexpressions if {capture, all_names} is given as an option to re:run/3. You can therefore create a name-to-value mapping from the result of re:run/3 like this:
1> {ok,MP} = re:compile("(?<A>A)|(?<B>B)|(?<C>C)"). {ok,{re_pattern,3,0,0, <<69,82,67,80,119,0,0,0,0,0,0,0,1,0,0,0,255,255,255,255, 255,255,...>>}} 2> {namelist, N} = re:inspect(MP,namelist). {namelist,[<<"A">>,<<"B">>,<<"C">>]} 3> {match,L} = re:run("AA",MP,[{capture,all_names,binary}]). {match,[<<"A">>,<<>>,<<>>]} 4> NameMap = lists:zip(N,L). [{<<"A">>,<<"A">>},{<<"B">>,<<>>},{<<"C">>,<<>>}]More items are expected to be added in the future.
run(Subject, RE) -> {match, Captured} | nomatch
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata()
Captured = [CaptureData]
CaptureData = {integer(), integer()}
The same as run(Subject,RE,[]).
run(Subject, RE, Options) -> {match, Captured} | match | nomatch | {error, ErrType}
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata() | unicode:charlist()
Options = [Option]
Option = anchored
| global
| notbol
| noteol
| notempty
| notempty_atstart
| report_errors
| {offset, integer() >= 0}
| {match_limit, integer() >= 0}
| {match_limit_recursion, integer() >= 0}
| {newline, NLSpec :: nl_spec()}
| bsr_anycrlf
| bsr_unicode
| {capture, ValueSpec}
| {capture, ValueSpec, Type}
| CompileOpt
Type = index | list | binary
ValueSpec = all
| all_but_first
| all_names
| first
| none
| ValueList
ValueList = [ValueID]
ValueID = integer() | string() | atom()
CompileOpt = compile_option()
CaptureData = {integer(), integer()}
| ListConversionData
| binary()
ListConversionData = string()
| {error, string(), binary()}
| {incomplete, string(), binary()}
ErrType = match_limit
| match_limit_recursion
| {compile, CompileErr}
CompileErr =
{ErrString :: string(), Position :: integer() >= 0}
| global
| notbol
| noteol
| notempty
| notempty_atstart
| report_errors
| {offset, integer() >= 0}
| {match_limit, integer() >= 0}
| {match_limit_recursion, integer() >= 0}
| {newline, NLSpec :: nl_spec()}
| bsr_anycrlf
| bsr_unicode
| {capture, ValueSpec}
| {capture, ValueSpec, Type}
| CompileOpt
| all_but_first
| all_names
| first
| none
| ValueList
See compile/2 above.
Captured = [CaptureData] | [[CaptureData]]
| ListConversionData
| binary()
| {error, string(), binary()}
| {incomplete, string(), binary()}
| match_limit_recursion
| {compile, CompileErr}
{ErrString :: string(), Position :: integer() >= 0}
Executes a regexp matching, returning match/{match, Captured} or
nomatch. The regular expression can be given either as iodata()
in which case it is automatically compiled (as by re:compile/2) and
executed, or as a pre-compiled mp() in which case it is executed
against the subject directly.
When compilation is involved, the exception badarg is thrown if a
compilation error occurs. Call re:compile/2 to get information about
the location of the error in the regular expression.
If the regular expression is previously compiled, the option list can only
contain the options anchored, global, notbol,
noteol, report_errors, notempty, notempty_atstart,
{offset, integer() >= 0}, {match_limit, integer() >= 0},
{match_limit_recursion, integer() >= 0}, {newline, NLSpec}
and {capture, ValueSpec}/{capture, ValueSpec, Type}. Otherwise all
options valid for the re:compile/2 function are allowed as well.
Options allowed both for compilation and execution of a match, namely
anchored and {newline, NLSpec}, will affect both the compilation
and execution if present together with a non pre-compiled regular expression.
If the regular expression was previously compiled with the option
unicode, the Subject should be provided as a valid Unicode
charlist(), otherwise any iodata() will do. If compilation is
involved and the option unicode is given, both the Subject and
the regular expression should be given as valid Unicode charlists().
The {capture, ValueSpec}/{capture, ValueSpec, Type} defines what to
return from the function upon successful matching. The capture tuple
may contain both a value specification telling which of the captured
substrings are to be returned, and a type specification, telling how captured
substrings are to be returned (as index tuples, lists or binaries). The
capture option makes the function quite flexible and powerful. The
different options are described in detail below.
If the capture options describe that no substring capturing at all is to be done
( {capture, none}), the function will return the single atom
match upon successful matching, otherwise the tuple {match,
ValueList} is returned. Disabling capturing can be done either by
specifying none or an empty list as ValueSpec.
The report_errors option adds the possibility that an error tuple is
returned. The tuple will either indicate a matching error ( match_limit
or match_limit_recursion) or a compilation error, where the error tuple
has the format {error, {compile, CompileErr}}. Note that if the option
report_errors is not given, the function never returns error tuples,
but will report compilation errors as a badarg exception and failed matches
due to exceeded match limits simply as nomatch.
The options relevant for execution are:
The options solely affecting the compilation step are described in the
re:compile/2 function.
- anchored:
- Limits re:run/3 to matching at the first matching position. If a pattern was compiled with anchored, or turned out to be anchored by virtue of its contents, it cannot be made unanchored at matching time, hence there is no unanchored option.
- global:
- Implements global (repetitive) search (the g flag in Perl). Each match is returned as a separate list() containing the specific match as well as any matching subexpressions (or as specified by the capture option). The Captured part of the return value will hence be a list() of list()s when this option is given.
The interaction of the global option with a regular expression which matches an
empty string surprises some users. When the global option is given,
re:run/3 handles empty matches in the same way as Perl: a zero-length
match at any point will be retried with the options [anchored,
notempty_atstart] as well. If that search gives a result of length > 0,
the result is included. For example:
re:run("cat","(|at)",[global]).
The following matching will be performed:
- At offset 0:
- The regexp (|at) will first match at the initial position of the string cat, giving the result set [{0,0},{0,0}] (the second {0,0} is due to the subexpression marked by the parentheses). As the length of the match is 0, we don't advance to the next position yet.
- At offset 0 with [anchored, notempty_atstart]:
-
The search is retried with the options [anchored, notempty_atstart] at the same position, which does not give any interesting result of longer length, so the search position is now advanced to the next character ( a).
- At offset 1:
- This time, the search results in [{1,0},{1,0}], so this search will also be repeated with the extra options.
- At offset 1 with [anchored, notempty_atstart]:
- Now the ab alternative is found and the result will be [{1,2},{1,2}]. The result is added to the list of results and the position in the search string is advanced two steps.
- At offset 3:
- The search now once again matches the empty string, giving [{3,0},{3,0}].
- At offset 1 with [anchored, notempty_atstart]:
- This will give no result of length > 0 and we are at the last position, so the global search is complete.
The result of the call is:
{match,[[{0,0},{0,0}],[{1,0},{1,0}],[{1,2},{1,2}],[{3,0},{3,0}]]}
- notempty:
- An empty string is not considered to be a valid match if this option is given. If there are alternatives in the pattern, they are tried. If all the alternatives match the empty string, the entire match fails. For example, if the pattern
a?b?
is applied to a string not beginning with "a" or "b", it
would normally match the empty string at the start of the subject. With the
notempty option, this match is not valid, so re:run/3 searches further
into the string for occurrences of "a" or "b".
- notempty_atstart:
- This is like notempty, except that an empty string match that is not at the start of the subject is permitted. If the pattern is anchored, such a match can occur only if the pattern contains \K.
Perl has no direct equivalent of notempty or notempty_atstart, but
it does make a special case of a pattern match of the empty string within its
split() function, and when using the /g modifier. It is possible to emulate
Perl's behavior after matching a null string by first trying the match again
at the same offset with notempty_atstart and anchored, and then,
if that fails, by advancing the starting offset (see below) and trying an
ordinary match again.
- notbol:
- This option specifies that the first character of the subject string is not the beginning of a line, so the circumflex metacharacter should not match before it. Setting this without multiline (at compile time) causes circumflex never to match. This option only affects the behavior of the circumflex metacharacter. It does not affect \\A.
- noteol:
- This option specifies that the end of the subject string is not the end of a line, so the dollar metacharacter should not match it nor (except in multiline mode) a newline immediately before it. Setting this without multiline (at compile time) causes dollar never to match. This option affects only the behavior of the dollar metacharacter. It does not affect \\Z or \\z.
- report_errors:
- This option gives better control of the error handling in re:run/3. When it is given, compilation errors (if the regular expression isn't already compiled) as well as run-time errors are explicitly returned as an error tuple.
The possible run-time errors are:
- match_limit:
- The PCRE library sets a limit on how many times the internal match function can be called. The default value for this is 10000000 in the library compiled for Erlang. If {error, match_limit} is returned, it means that the execution of the regular expression has reached this limit. Normally this is to be regarded as a nomatch, which is the default return value when this happens, but by specifying report_errors, you will get informed when the match fails due to to many internal calls.
- match_limit_recursion:
- This error is very similar to match_limit, but occurs when the internal match function of PCRE is "recursively" called more times than the "match_limit_recursion" limit, which is by default 10000000 as well. Note that as long as the match_limit and match_limit_default values are kept at the default values, the match_limit_recursion error can not occur, as the match_limit error will occur before that (each recursive call is also a call, but not vice versa). Both limits can however be changed, either by setting limits directly in the regular expression string (see reference section below) or by giving options to re:run/3
It is important to understand that what is referred to as "recursion"
when limiting matches is not actually recursion on the C stack of the Erlang
machine, neither is it recursion on the Erlang process stack. The version of
PCRE compiled into the Erlang VM uses machine "heap" memory to store
values that needs to be kept over recursion in regular expression
matches.
- {match_limit, integer() >= 0}:
- This option limits the execution time of a match in an implementation-specific way. It is described in the following way by the PCRE documentation:
The match_limit field provides a means of preventing PCRE from using up a vast amount of resources when running patterns that are not going to match, but which have a very large number of possibilities in their search trees. The classic example is a pattern that uses nested unlimited repeats. Internally, pcre_exec() uses a function called match(), which it calls repeatedly (sometimes recursively). The limit set by match_limit is imposed on the number of times this function is called during a match, which has the effect of limiting the amount of backtracking that can take place. For patterns that are not anchored, the count restarts from zero for each position in the subject string.
This means that runaway regular expression matches can fail faster if the limit
is lowered using this option. The default value compiled into the Erlang
virtual machine is 10000000
Note:
This option does in no way affect the execution of the Erlang virtual machine in
terms of "long running BIF's". re:run always give control
back to the scheduler of Erlang processes at intervals that ensures the real
time properties of the Erlang system.
- {match_limit_recursion, integer() >= 0}:
- This option limits the execution time and memory consumption of a match in an implementation-specific way, very similar to match_limit. It is described in the following way by the PCRE documentation:
The match_limit_recursion field is similar to match_limit, but instead of limiting the total number of times that match() is called, it limits the depth of recursion. The recursion depth is a smaller number than the total number of calls, because not all calls to match() are recursive. This limit is of use only if it is set smaller than match_limit. Limiting the recursion depth limits the amount of machine stack that can be used, or, when PCRE has been compiled to use memory on the heap instead of the stack, the amount of heap memory that can be used.
The Erlang virtual machine uses a PCRE library where heap memory is used when
regular expression match recursion happens, why this limits the usage of
machine heap, not C stack.
Specifying a lower value may result in matches with deep recursion failing, when
they should actually have matched:
1> re:run("aaaaaaaaaaaaaz","(a+)*z"). {match,[{0,14},{0,13}]} 2> re:run("aaaaaaaaaaaaaz","(a+)*z",[{match_limit_recursion,5}]). nomatch 3> re:run("aaaaaaaaaaaaaz","(a+)*z",[{match_limit_recursion,5},report_errors]). {error,match_limit_recursion}
This option, as well as the match_limit option should only be used in
very rare cases. Understanding of the PCRE library internals is recommended
before tampering with these limits.
- {offset, integer() >= 0}:
- Start matching at the offset (position) given in the subject string. The offset is zero-based, so that the default is {offset,0} (all of the subject string).
- {newline, NLSpec}:
- Override the default definition of a newline in the subject string, which is LF (ASCII 10) in Erlang.
- cr:
- Newline is indicated by a single character CR (ASCII 13)
- lf:
- Newline is indicated by a single character LF (ASCII 10), the default
- crlf:
- Newline is indicated by the two-character CRLF (ASCII 13 followed by ASCII 10) sequence.
- anycrlf:
- Any of the three preceding sequences should be recognized.
- any:
- Any of the newline sequences above, plus the Unicode sequences VT (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029).
- bsr_anycrlf:
- Specifies specifically that \R is to match only the cr, lf or crlf sequences, not the Unicode specific newline characters. (overrides compilation option)
- bsr_unicode:
- Specifies specifically that \R is to match all the Unicode newline characters (including crlf etc, the default).(overrides compilation option)
- {capture, ValueSpec}/{capture, ValueSpec, Type}:
- Specifies which captured substrings are returned and in what format. By default, re:run/3 captures all of the matching part of the substring as well as all capturing subpatterns (all of the pattern is automatically captured). The default return type is (zero-based) indexes of the captured parts of the string, given as {Offset,Length} pairs (the index Type of capturing).
As an example of the default behavior, the following call:
re:run("ABCabcdABC","abcd",[]).
returns, as first and only captured string the matching part of the subject
("abcd" in the middle) as a index pair {3,4}, where character
positions are zero based, just as in offsets. The return value of the call
above would then be:
{match,[{3,4}]}
Another (and quite common) case is where the regular expression matches all of
the subject, as in:
re:run("ABCabcdABC",".*abcd.*",[]).
where the return value correspondingly will point out all of the string,
beginning at index 0 and being 10 characters long:
{match,[{0,10}]}
If the regular expression contains capturing subpatterns, like in the following
case:
re:run("ABCabcdABC",".*(abcd).*",[]).
all of the matched subject is captured, as well as the captured
substrings:
{match,[{0,10},{3,4}]}
the complete matching pattern always giving the first return value in the list
and the rest of the subpatterns being added in the order they occurred in the
regular expression.
The capture tuple is built up as follows:
- ValueSpec:
- Specifies which captured (sub)patterns are to be returned. The ValueSpec can either be an atom describing a predefined set of return values, or a list containing either the indexes or the names of specific subpatterns to return.
The predefined sets of subpatterns are:
- all:
- All captured subpatterns including the complete matching string. This is the default.
- all_names:
- All named subpatterns in the regular expression, as if a list() of all the names in alphabetical order was given. The list of all names can also be retrieved with the inspect/2 function.
- first:
- Only the first captured subpattern, which is always the complete matching part of the subject. All explicitly captured subpatterns are discarded.
- all_but_first:
- All but the first matching subpattern, i.e. all explicitly captured subpatterns, but not the complete matching part of the subject string. This is useful if the regular expression as a whole matches a large part of the subject, but the part you're interested in is in an explicitly captured subpattern. If the return type is list or binary, not returning subpatterns you're not interested in is a good way to optimize.
- none:
- Do not return matching subpatterns at all, yielding the single atom match as the return value of the function when matching successfully instead of the {match, list()} return. Specifying an empty list gives the same behavior.
The value list is a list of indexes for the subpatterns to return, where index 0
is for all of the pattern, and 1 is for the first explicit capturing
subpattern in the regular expression, and so forth. When using named captured
subpatterns (see below) in the regular expression, one can use atom()s
or string()s to specify the subpatterns to be returned. For example,
consider the regular expression:
".*(abcd).*"
matched against the string "ABCabcdABC", capturing only the
"abcd" part (the first explicit subpattern):
re:run("ABCabcdABC",".*(abcd).*",[{capture,[1]}]).
The call will yield the following result:
{match,[{3,4}]}
as the first explicitly captured subpattern is "(abcd)", matching
"abcd" in the subject, at (zero-based) position 3, of length
4.
Now consider the same regular expression, but with the subpattern explicitly
named 'FOO':
".*(?<FOO>abcd).*"
With this expression, we could still give the index of the subpattern with the
following call:
re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,[1]}]).
giving the same result as before. But, since the subpattern is named, we can
also specify its name in the value list:
re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,['FOO']}]).
which would yield the same result as the earlier examples, namely:
{match,[{3,4}]}
The values list might specify indexes or names not present in the regular
expression, in which case the return values vary depending on the type. If the
type is index, the tuple {-1,0} is returned for values having no
corresponding subpattern in the regexp, but for the other types (
binary and list), the values are the empty binary or list
respectively.
- Type:
- Optionally specifies how captured substrings are to be returned. If omitted, the default of index is used. The Type can be one of the following:
- index:
- Return captured substrings as pairs of byte indexes into the subject string and length of the matching string in the subject (as if the subject string was flattened with iolist_to_binary/1 or unicode:characters_to_binary/2 prior to matching). Note that the unicode option results in byte-oriented indexes in a (possibly virtual) UTF-8 encoded binary. A byte index tuple {0,2} might therefore represent one or two characters when unicode is in effect. This might seem counter-intuitive, but has been deemed the most effective and useful way to way to do it. To return lists instead might result in simpler code if that is desired. This return type is the default.
- list:
- Return matching substrings as lists of characters (Erlang string()s). It the unicode option is used in combination with the \C sequence in the regular expression, a captured subpattern can contain bytes that are not valid UTF-8 (\C matches bytes regardless of character encoding). In that case the list capturing may result in the same types of tuples that unicode:characters_to_list/2 can return, namely three-tuples with the tag incomplete or error, the successfully converted characters and the invalid UTF-8 tail of the conversion as a binary. The best strategy is to avoid using the \C sequence when capturing lists.
- binary:
- Return matching substrings as binaries. If the unicode option is used, these binaries are in UTF-8. If the \C sequence is used together with unicode the binaries may be invalid UTF-8.
In general, subpatterns that were not assigned a value in the match are returned
as the tuple {-1,0} when type is index. Unassigned
subpatterns are returned as the empty binary or list, respectively, for other
return types. Consider the regular expression:
".*((?<FOO>abdd)|a(..d)).*"
There are three explicitly capturing subpatterns, where the opening parenthesis
position determines the order in the result, hence
((?<FOO>abdd)|a(..d)) is subpattern index 1,
(?<FOO>abdd) is subpattern index 2 and (..d) is subpattern
index 3. When matched against the following string:
"ABCabcdABC"
the subpattern at index 2 won't match, as "abdd" is not present in the
string, but the complete pattern matches (due to the alternative
a(..d). The subpattern at index 2 is therefore unassigned and the
default return value will be:
{match,[{0,10},{3,4},{-1,0},{4,3}]}
Setting the capture Type to binary would give the following:
{match,[<<"ABCabcdABC">>,<<"abcd">>,<<>>,<<"bcd">>]}
where the empty binary ( <<>>) represents the unassigned
subpattern. In the binary case, some information about the matching is
therefore lost, the <<>> might just as well be an empty
string captured.
If differentiation between empty matches and non existing subpatterns is
necessary, use the type index and do the conversion to the final
type in Erlang code.
When the option global is given, the capture specification affects
each match separately, so that:
re:run("cacb","c(a|b)",[global,{capture,[1],list}]).
gives the result:
{match,[["a"],["b"]]}
replace(Subject, RE, Replacement) -> iodata() | unicode:charlist()
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata()
Replacement = iodata() | unicode:charlist()
The same as replace(Subject,RE,Replacement,[]).
replace(Subject, RE, Replacement, Options) -> iodata() | unicode:charlist()
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata() | unicode:charlist()
Replacement = iodata() | unicode:charlist()
Options = [Option]
Option = anchored
| global
| notbol
| noteol
| notempty
| notempty_atstart
| {offset, integer() >= 0}
| {newline, NLSpec}
| bsr_anycrlf
| {match_limit, integer() >= 0}
| {match_limit_recursion, integer() >= 0}
| bsr_unicode
| {return, ReturnType}
| CompileOpt
ReturnType = iodata | list | binary
CompileOpt = compile_option()
NLSpec = cr | crlf | lf | anycrlf | any
| global
| notbol
| noteol
| notempty
| notempty_atstart
| {offset, integer() >= 0}
| {newline, NLSpec}
| bsr_anycrlf
| {match_limit, integer() >= 0}
| {match_limit_recursion, integer() >= 0}
| bsr_unicode
| {return, ReturnType}
| CompileOpt
Replaces the matched part of the Subject string with the contents of
Replacement.
The permissible options are the same as for re:run/3, except that the
capture option is not allowed. Instead a {return, ReturnType} is
present. The default return type is iodata, constructed in a way to
minimize copying. The iodata result can be used directly in many
I/O-operations. If a flat list() is desired, specify {return,
list} and if a binary is preferred, specify {return, binary}.
As in the re:run/3 function, an mp() compiled with the
unicode option requires the Subject to be a Unicode
charlist(). If compilation is done implicitly and the unicode
compilation option is given to this function, both the regular expression and
the Subject should be given as valid Unicode charlist()s.
The replacement string can contain the special character &, which
inserts the whole matching expression in the result, and the special sequence
\N (where N is an integer > 0), \gN or \g{N}
resulting in the subexpression number N will be inserted in the result. If no
subexpression with that number is generated by the regular expression, nothing
is inserted.
To insert an & or \ in the result, precede it with a \.
Note that Erlang already gives a special meaning to \ in literal
strings, so a single \ has to be written as "\\" and
therefore a double \ as "\\\\". Example:
re:replace("abcd","c","[&]",[{return,list}]).gives
"ab[c]d"while
re:replace("abcd","c","[\\&]",[{return,list}]).gives
"ab[&]d"As with re:run/3, compilation errors raise the badarg exception, re:compile/2 can be used to get more information about the error.
split(Subject, RE) -> SplitList
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata()
SplitList = [iodata() | unicode:charlist()]
The same as split(Subject,RE,[]).
split(Subject, RE, Options) -> SplitList
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata() | unicode:charlist()
Options = [Option]
Option = anchored
| notbol
| noteol
| notempty
| notempty_atstart
| {offset, integer() >= 0}
| {newline, nl_spec()}
| {match_limit, integer() >= 0}
| {match_limit_recursion, integer() >= 0}
| bsr_anycrlf
| bsr_unicode
| {return, ReturnType}
| {parts, NumParts}
| group
| trim
| CompileOpt
NumParts = integer() >= 0 | infinity
ReturnType = iodata | list | binary
CompileOpt = compile_option()
GroupedRetData = [RetData]
RetData = iodata() | unicode:charlist() | binary() | list()
| notbol
| noteol
| notempty
| notempty_atstart
| {offset, integer() >= 0}
| {newline, nl_spec()}
| {match_limit, integer() >= 0}
| {match_limit_recursion, integer() >= 0}
| bsr_anycrlf
| bsr_unicode
| {return, ReturnType}
| {parts, NumParts}
| group
| trim
| CompileOpt
See compile/2 above.
SplitList = [RetData] | [GroupedRetData]
This function splits the input into parts by finding tokens according to the
regular expression supplied.
The splitting is done basically by running a global regexp match and dividing
the initial string wherever a match occurs. The matching part of the string is
removed from the output.
As in the re:run/3 function, an mp() compiled with the
unicode option requires the Subject to be a Unicode
charlist(). If compilation is done implicitly and the unicode
compilation option is given to this function, both the regular expression and
the Subject should be given as valid Unicode charlist()s.
The result is given as a list of "strings", the preferred datatype
given in the return option (default iodata).
If subexpressions are given in the regular expression, the matching
subexpressions are returned in the resulting list as well. An example:
re:split("Erlang","[ln]",[{return,list}]).will yield the result:
["Er","a","g"]while
re:split("Erlang","([ln])",[{return,list}]).will yield
["Er","l","a","n","g"]The text matching the subexpression (marked by the parentheses in the regexp) is inserted in the result list where it was found. In effect this means that concatenating the result of a split where the whole regexp is a single subexpression (as in the example above) will always result in the original string. As there is no matching subexpression for the last part in the example (the "g"), there is nothing inserted after that. To make the group of strings and the parts matching the subexpressions more obvious, one might use the group option, which groups together the part of the subject string with the parts matching the subexpressions when the string was split:
re:split("Erlang","([ln])",[{return,list},group]).gives:
[["Er","l"],["a","n"],["g"]]Here the regular expression matched first the "l", causing "Er" to be the first part in the result. When the regular expression matched, the (only) subexpression was bound to the "l", so the "l" is inserted in the group together with "Er". The next match is of the "n", making "a" the next part to be returned. Since the subexpression is bound to the substring "n" in this case, the "n" is inserted into this group. The last group consists of the rest of the string, as no more matches are found. By default, all parts of the string, including the empty strings, are returned from the function. For example:
re:split("Erlang","[lg]",[{return,list}]).will return:
["Er","an",[]]since the matching of the "g" in the end of the string leaves an empty rest which is also returned. This behaviour differs from the default behaviour of the split function in Perl, where empty strings at the end are by default removed. To get the "trimming" default behavior of Perl, specify trim as an option:
re:split("Erlang","[lg]",[{return,list},trim]).The result will be:
["Er","an"]The "trim" option in effect says; "give me as many parts as possible except the empty ones", which might be useful in some circumstances. You can also specify how many parts you want, by specifying {parts,N }:
re:split("Erlang","[lg]",[{return,list},{parts,2}]).This will give:
["Er","ang"]Note that the last part is "ang", not "an", as we only specified splitting into two parts, and the splitting stops when enough parts are given, which is why the result differs from that of trim. More than three parts are not possible with this indata, so
re:split("Erlang","[lg]",[{return,list},{parts,4}]).will give the same result as the default, which is to be viewed as "an infinite number of parts". Specifying 0 as the number of parts gives the same effect as the option trim. If subexpressions are captured, empty subexpression matches at the end are also stripped from the result if trim or {parts,0} is specified. If you are familiar with Perl, the trim behaviour corresponds exactly to the Perl default, the {parts,N} where N is a positive integer corresponds exactly to the Perl behaviour with a positive numerical third parameter and the default behaviour of re:split/3 corresponds to that when the Perl routine is given a negative integer as the third parameter. Summary of options not previously described for the re:run/3 function:
- {return,ReturnType}:
- Specifies how the parts of the original string are presented in the result list. The possible types are:
- iodata:
- The variant of iodata() that gives the least copying of data with the current implementation (often a binary, but don't depend on it).
- binary:
- All parts returned as binaries.
- list:
- All parts returned as lists of characters ("strings").
- group:
- Groups together the part of the string with the parts of the string matching the subexpressions of the regexp.
The return value from the function will in this case be a list() of
list()s. Each sublist begins with the string picked out of the subject
string, followed by the parts matching each of the subexpressions in order of
occurrence in the regular expression.
- {parts,N}:
- Specifies the number of parts the subject string is to be split into.
The number of parts should be a positive integer for a specific maximum on the
number of parts and infinity for the maximum number of parts possible
(the default). Specifying {parts,0} gives as many parts as possible
disregarding empty parts at the end, the same as specifying trim
- trim:
- Specifies that empty parts at the end of the result list are to be disregarded. The same as specifying {parts,0}. This corresponds to the default behaviour of the split built in function in Perl.
PERL LIKE REGULAR EXPRESSIONS SYNTAX¶
The following sections contain reference material for the regular expressions used by this module. The regular expression reference is based on the PCRE documentation, with changes in cases where the re module behaves differently to the PCRE library.PCRE REGULAR EXPRESSION DETAILS¶
The syntax and semantics of the regular expressions that are supported by PCRE are described in detail below. Perl's regular expressions are described in its own documentation, and regular expressions in general are covered in a number of books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers regular expressions in great detail. This description of PCRE's regular expressions is intended as reference material. The reference material is divided into the following sections:- *
- Special start-of-pattern items
- *
- Characters and metacharacters
- *
- Backslash
- *
- Circumflex and dollar
- *
- Full stop (period, dot) and \N
- *
- Matching a single data unit
- *
- Square brackets and character classes
- *
- POSIX character classes
- *
- Vertical bar
- *
- Internal option setting
- *
- Subpatterns
- *
- Duplicate subpattern numbers
- *
- Named subpatterns
- *
- Repetition
- *
- Atomic grouping and possessive quantifiers
- *
- Back references
- *
- Assertions
- *
- Conditional subpatterns
- *
- Comments
- *
- Recursive patterns
- *
- Subpatterns as subroutines
- *
- Oniguruma subroutine syntax
- *
- Backtracking control
SPECIAL START-OF-PATTERN ITEMS¶
A number of options that can be passed to re:compile/2 can also be set by special items at the start of a pattern. These are not Perl-compatible, but are provided to make these options accessible to pattern writers who are not able to change the program that processes the pattern. Any number of these items may appear, but they must all be together right at the start of the pattern string, and the letters must be in upper case. UTF support Unicode support is basically UTF-8 based. To use Unicode characters, you either call re:compile/2/re:run/3 with the unicode option, or the pattern must start with one of these special sequences: (*UTF8) (*UTF) Both options give the same effect, the input string is interpreted as UTF-8. Note that with these instructions, the automatic conversion of lists to UTF-8 is not performed by the re functions, why using these options is not recommended. Add the unicode option when running re:compile/2 instead. Some applications that allow their users to supply patterns may wish to restrict them to non-UTF data for security reasons. If the never_utf option is set at compile time, (*UTF) etc. are not allowed, and their appearance causes an error. Unicode property support Another special sequence that may appear at the start of a pattern is (*UCP) This has the same effect as setting the ucp option: it causes sequences such as \d and \w to use Unicode properties to determine character types, instead of recognizing only characters with codes less than 128 via a lookup table. Disabling start-up optimizations If a pattern starts with (*NO_START_OPT), it has the same effect as setting the no_Start_optimize option at compile time. Newline conventions PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (linefeed) character, the two-character sequence CRLF , any of the three preceding, or any Unicode newline sequence. It is also possible to specify a newline convention by starting a pattern string with one of the following five sequences:- (*CR):
- carriage return
- (*LF):
- linefeed
- (*CRLF):
- carriage return, followed by linefeed
- (*ANYCRLF):
- any of the three above
- (*ANY):
- all Unicode newline sequences
CHARACTERS AND METACHARACTERS¶
A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern The quick brown fox matches a portion of a subject string that is identical to itself. When caseless matching is specified (the caseless option), letters are matched independently of case. The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of metacharacters, which do not stand for themselves but instead are interpreted in some special way. There are two different sets of metacharacters: those that are recognized anywhere in the pattern except within square brackets, and those that are recognized within square brackets. Outside square brackets, the metacharacters are as follows:- \:
- general escape character with several uses
- ^:
- assert start of string (or line, in multiline mode)
- $:
- assert end of string (or line, in multiline mode)
- .:
- match any character except newline (by default)
- [:
- start character class definition
- |:
- start of alternative branch
- (:
- start subpattern
- ):
- end subpattern
- ?:
- extends the meaning of (, also 0 or 1 quantifier, also quantifier minimizer
- *:
- 0 or more quantifier
- +:
- 1 or more quantifier, also "possessive quantifier"
- {:
- start min/max quantifier
- \:
- general escape character
- ^:
- negate the class, but only if the first character
- -:
- indicates character range
- [:
- POSIX character class (only if followed by POSIX syntax)
- ]:
- terminates the character class
BACKSLASH¶
The backslash character has several uses. Firstly, if it is followed by a character that is not a number or a letter, it takes away any special meaning that character may have. This use of backslash as an escape character applies both inside and outside character classes. For example, if you want to match a * character, you write \* in the pattern. This escaping action applies whether or not the following character would otherwise be interpreted as a metacharacter, so it is always safe to precede a non-alphanumeric with backslash to specify that it stands for itself. In particular, if you want to match a backslash, you write \\. In unicode mode, only ASCII numbers and letters have any special meaning after a backslash. All other characters (in particular, those whose codepoints are greater than 127) are treated as literals. If a pattern is compiled with the extended option, white space in the pattern (other than in a character class) and characters between a # outside a character class and the next newline are ignored. An escaping backslash can be used to include a white space or # character as part of the pattern. If you want to remove the special meaning from a sequence of characters, you can do so by putting them between \Q and \E. This is different from Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable interpolation. Note the following examples:Pattern PCRE matches Perl matches \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyzThe \Q...\E sequence is recognized both inside and outside character classes. An isolated \E that is not preceded by \Q is ignored. If \Q is not followed by \E later in the pattern, the literal interpretation continues to the end of the pattern (that is, \E is assumed at the end). If the isolated \Q is inside a character class, this causes an error, because the character class is not terminated. Non-printing characters A second use of backslash provides a way of encoding non-printing characters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters, apart from the binary zero that terminates a pattern, but when a pattern is being prepared by text editing, it is often easier to use one of the following escape sequences than the binary character it represents:
- \a:
- alarm, that is, the BEL character (hex 07)
- \cx:
- "control-x", where x is any ASCII character
- \e :
- escape (hex 1B)
- \f:
- form feed (hex 0C)
- \n:
- linefeed (hex 0A)
- \r:
- carriage return (hex 0D)
- \t :
- tab (hex 09)
- \ddd:
- character with octal code ddd, or back reference
- \xhh :
- character with hex code hh
- \x{hhh..}:
- character with hex code hhh..
- 8-bit non-Unicode mode:
- less than 0x100
- 8-bit UTF-8 mode:
- less than 0x10ffff and a valid codepoint
- \040:
- is another way of writing a ASCII space
- \40:
- is the same, provided there are fewer than 40 previous capturing subpatterns
- \7:
- is always a back reference
- \11:
-
might be a back reference, or another way of writing a tab
- \011:
- is always a tab
- \0113:
- is a tab followed by the character "3"
- \113:
- might be a back reference, otherwise the character with octal code 113
- \377:
- might be a back reference, otherwise the value 255 (decimal)
- \81:
- is either a back reference, or a binary zero followed by the two characters "8" and "1"
- \d:
- any decimal digit
- \D:
- any character that is not a decimal digit
- \h:
- any horizontal white space character
- \H:
- any character that is not a horizontal white space character
- \s:
- any white space character
- \S:
- any character that is not a white space character
- \v:
- any vertical white space character
- \V:
- any character that is not a vertical white space character
- \w:
- any "word" character
- \W:
- any "non-word" character
- \d:
- any character that \p{Nd} matches (decimal digit)
- \s:
- any character that \p{Z} matches, plus HT, LF, FF, CR)
-
\w: - any character that \p{L} or \p{N} matches, plus underscore)
- U+0009:
- Horizontal tab (HT)
- U+0020:
- Space
- U+00A0:
- Non-break space
- U+1680:
- Ogham space mark
- U+180E:
- Mongolian vowel separator
- U+2000:
- En quad
- U+2001:
- Em quad
- U+2002:
- En space
- U+2003:
- Em space
- U+2004:
- Three-per-em space
- U+2005:
- Four-per-em space
- U+2006:
- Six-per-em space
- U+2007:
- Figure space
- U+2008:
- Punctuation space
- U+2009:
- Thin space
- U+200A:
- Hair space
- U+202F:
- Narrow no-break space
- U+205F:
- Medium mathematical space
- U+3000:
- Ideographic space
- U+000A:
- Linefeed (LF)
- U+000B:
- Vertical tab (VT)
- U+000C:
- Form feed (FF)
- U+000D:
- Carriage return (CR)
- U+0085:
- Next line (NEL)
- U+2028:
- Line separator
- U+2029:
- Paragraph separator
- \p{xx}:
- a character with the xx property
- \P{xx}:
- a character without the xx property
- \X:
- a Unicode extended grapheme cluster
- *
- Arabic
- *
- Armenian
- *
- Avestan
- *
- Balinese
- *
- Bamum
- *
- Batak
- *
- Bengali
- *
- Bopomofo
- *
- Braille
- *
- Buginese
- *
- Buhid
- *
- Canadian_Aboriginal
- *
- Carian
- *
- Chakma
- *
- Cham
- *
- Cherokee
- *
- Common
- *
- Coptic
- *
- Cuneiform
- *
- Cypriot
- *
- Cyrillic
- *
- Deseret
- *
- Devanagari
- *
- Egyptian_Hieroglyphs
- *
- Ethiopic
- *
- Georgian
- *
- Glagolitic
- *
- Gothic
- *
- Greek
- *
- Gujarati
- *
- Gurmukhi
- *
- Han
- *
- Hangul
- *
- Hanunoo
- *
- Hebrew
- *
- Hiragana
- *
- Imperial_Aramaic
- *
- Inherited
- *
- Inscriptional_Pahlavi
- *
- Inscriptional_Parthian
- *
- Javanese
- *
- Kaithi
- *
- Kannada
- *
- Katakana
- *
- Kayah_Li
- *
- Kharoshthi
- *
- Khmer
- *
- Lao
- *
- Latin
- *
- Lepcha
- *
- Limbu
- *
- Linear_B
- *
- Lisu
- *
- Lycian
- *
- Lydian
- *
- Malayalam
- *
- Mandaic
- *
- Meetei_Mayek
- *
- Meroitic_Cursive
- *
- Meroitic_Hieroglyphs
- *
- Miao
- *
- Mongolian
- *
- Myanmar
- *
- New_Tai_Lue
- *
- Nko
- *
- Ogham
- *
- Old_Italic
- *
- Old_Persian
- *
- Oriya
- *
- Old_South_Arabian
- *
- Old_Turkic
- *
- Ol_Chiki
- *
- Osmanya
- *
- Phags_Pa
- *
- Phoenician
- *
- Rejang
- *
- Runic
- *
- Samaritan
- *
- Saurashtra
- *
- Sharada
- *
- Shavian
- *
- Sinhala
- *
- Sora_Sompeng
- *
- Sundanese
- *
- Syloti_Nagri
- *
- Syriac
- *
- Tagalog
- *
- Tagbanwa
- *
- Tai_Le
- *
- Tai_Tham
- *
- Tai_Viet
- *
- Takri
- *
- Tamil
- *
- Telugu
- *
- Thaana
- *
- Thai
- *
- Tibetan
- *
- Tifinagh
- *
- Ugaritic
- *
- Vai
- *
- Yi
- *
- \p{L}
- *
- \pL
- C:
- Other
- Cc:
- Control
- Cf:
- Format
- Cn:
- Unassigned
- Co:
- Private use
- Cs:
- Surrogate
- L:
- Letter
- Ll:
- Lower case letter
- Lm:
- Modifier letter
- Lo:
- Other letter
- Lt:
- Title case letter
- Lu:
- Upper case letter
- M:
- Mark
- Mc:
- Spacing mark
- Me:
- Enclosing mark
- Mn:
- Non-spacing mark
- N:
- Number
- Nd:
- Decimal number
- Nl:
- Letter number
- No:
- Other number
- P:
- Punctuation
- Pc:
- Connector punctuation
- Pd:
- Dash punctuation
- Pe:
- Close punctuation
- Pf:
- Final punctuation
- Pi:
- Initial punctuation
- Po:
- Other punctuation
- Ps:
- Open punctuation
- S:
- Symbol
- Sc:
- Currency symbol
- Sk:
- Modifier symbol
- Sm:
- Mathematical symbol
- So:
- Other symbol
- Z:
- Separator
- Zl:
- Line separator
- Zp:
- Paragraph separator
- Zs:
- Space separator
- 1.:
- End at the end of the subject string.
- 2.:
- Do not end between CR and LF; otherwise end after any control character.
- 3.:
- Do not break Hangul (a Korean script) syllable sequences. Hangul characters are of five types: L, V, T, LV, and LVT. An L character may be followed by an L, V, LV, or LVT character; an LV or V character may be followed by a V or T character; an LVT or T character may be follwed only by a T character.
- 4.:
- Do not end before extending characters or spacing marks. Characters with the "mark" property always have the "extend" grapheme breaking property.
- 5.:
- Do not end after prepend characters.
- 6.:
- Otherwise, end the cluster.
- Xan:
- Any alphanumeric character
- Xps:
- Any POSIX space character
- Xsp:
- Any Perl space character
- Xwd:
- Any Perl "word" character
- \b:
- matches at a word boundary
- \B:
- matches when not at a word boundary
- \A:
- matches at the start of the subject
- \Z:
- matches at the end of the subject also matches before a newline at the end of the subject
- \z:
- matches only at the end of the subject
- \G:
- matches at the first matching position in the subject
CIRCUMFLEX AND DOLLAR¶
The circumflex and dollar metacharacters are zero-width assertions. That is, they test for a particular condition being true without consuming any characters from the subject string. Outside a character class, in the default matching mode, the circumflex character is an assertion that is true only if the current matching point is at the start of the subject string. If the startoffset argument of re:run/3 is non-zero, circumflex can never match if the multiline option is unset. Inside a character class, circumflex has an entirely different meaning (see below). Circumflex need not be the first character of the pattern if a number of alternatives are involved, but it should be the first thing in each alternative in which it appears if the pattern is ever to match that branch. If all possible alternatives start with a circumflex, that is, if the pattern is constrained to match only at the start of the subject, it is said to be an "anchored" pattern. (There are also other constructs that can cause a pattern to be anchored.) The dollar character is an assertion that is true only if the current matching point is at the end of the subject string, or immediately before a newline at the end of the string (by default). Note, however, that it does not actually match the newline. Dollar need not be the last character of the pattern if a number of alternatives are involved, but it should be the last item in any branch in which it appears. Dollar has no special meaning in a character class. The meaning of dollar can be changed so that it matches only at the very end of the string, by setting the dollar_endonly option at compile time. This does not affect the \Z assertion. The meanings of the circumflex and dollar characters are changed if the multiline option is set. When this is the case, a circumflex matches immediately after internal newlines as well as at the start of the subject string. It does not match after a newline that ends the string. A dollar matches before any newlines in the string, as well as at the very end, when multiline is set. When newline is specified as the two-character sequence CRLF, isolated CR and LF characters do not indicate newlines. For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n represents a newline) in multiline mode, but not otherwise. Consequently, patterns that are anchored in single line mode because all branches start with ^ are not anchored in multiline mode, and a match for circumflex is possible when the startoffset argument of re:run/3 is non-zero. The dollar_endonly option is ignored if multiline is set. Note that the sequences \A, \Z, and \z can be used to match the start and end of the subject in both modes, and if all branches of a pattern start with \A it is always anchored, whether or not multiline is set.FULL STOP (PERIOD, DOT) AND \N¶
Outside a character class, a dot in the pattern matches any one character in the subject string except (by default) a character that signifies the end of a line. When a line ending is defined as a single character, dot never matches that character; when the two-character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, but otherwise it matches all characters (including isolated CRs and LFs). When any Unicode line endings are being recognized, dot does not match CR or LF or any of the other line ending characters. The behaviour of dot with regard to newlines can be changed. If the dotall option is set, a dot matches any one character, without exception. If the two-character sequence CRLF is present in the subject string, it takes two dots to match it. The handling of dot is entirely independent of the handling of circumflex and dollar, the only relationship being that they both involve newlines. Dot has no special meaning in a character class. The escape sequence \N behaves like a dot, except that it is not affected by the PCRE_DOTALL option. In other words, it matches any character except one that signifies the end of a line. Perl also uses \N to match characters by name; PCRE does not support this.MATCHING A SINGLE DATA UNIT¶
Outside a character class, the escape sequence \C matches any one data unit, whether or not a UTF mode is set. One data unit is one byte. Unlike a dot, \C always matches line-ending characters. The feature is provided in Perl in order to match individual bytes in UTF-8 mode, but it is unclear how it can usefully be used. Because \C breaks up characters into individual data units, matching one unit with \C in a UTF mode means that the rest of the string may start with a malformed UTF character. This has undefined results, because PCRE assumes that it is dealing with valid UTF strings. PCRE does not allow \C to appear in lookbehind assertions (described below) in a UTF mode, because this would make it impossible to calculate the length of the lookbehind. In general, the \C escape sequence is best avoided. However, one way of using it that avoids the problem of malformed UTF characters is to use a lookahead to check the length of the next character, as in this pattern, which could be used with a UTF-8 string (ignore white space and line breaks):(?| (?=[\x00-\x7f])(\C) | (?=[\x80-\x{7ff}])(\C)(\C) | (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))A group that starts with (?| resets the capturing parentheses numbers in each alternative (see "Duplicate Subpattern Numbers" below). The assertions at the start of each branch check the next UTF-8 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The character's individual bytes are then captured by the appropriate number of groups.
SQUARE BRACKETS AND CHARACTER CLASSES¶
An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not special by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square bracket causes a compile-time error. If a closing square bracket is required as a member of the class, it should be the first data character in the class (after an initial circumflex, if present) or escaped with a backslash. A character class matches a single character in the subject. In a UTF mode, the character may be more than one data unit long. A matched character must be in the set of characters defined by the class, unless the first character in the class definition is a circumflex, in which case the subject character must not be in the set defined by the class. If a circumflex is actually required as a member of the class, ensure it is not the first character, or escape it with a backslash. For example, the character class [aeiou] matches any lower case vowel, while [^aeiou] matches any character that is not a lower case vowel. Note that a circumflex is just a convenient notation for specifying the characters that are in the class by enumerating those that are not. A class that starts with a circumflex is not an assertion; it still consumes a character from the subject string, and therefore it fails if the current pointer is at the end of the string. In UTF-8 mode, characters with values greater than 255 (0xffff) can be included in a class as a literal string of data units, or by using the \x{ escaping mechanism. When caseless matching is set, any letters in a class represent both their upper case and lower case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a caseful version would. In a UTF mode, PCRE always understands the concept of case for characters whose values are less than 128, so caseless matching is always possible. For characters with higher values, the concept of case is supported if PCRE is compiled with Unicode property support, but not otherwise. If you want to use caseless matching in a UTF mode for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF support. Characters that might indicate line breaks are never treated in any special way when matching character classes, whatever line-ending sequence is in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class such as [^a] always matches one of these characters. The minus (hyphen) character can be used to specify a range of characters in a character class. For example, [d-m] matches any letter between d and m, inclusive. If a minus character is required in a class, it must be escaped with a backslash or appear in a position where it cannot be interpreted as indicating a range, typically as the first or last character in the class. It is not possible to have the literal character "]" as the end character of a range. A pattern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed by a literal string "46]", so it would match "W46]" or "-46]". However, if the "]" is escaped with a backslash it is interpreted as the end of range, so [W-\]46] is interpreted as a class containing a range followed by two other characters. The octal or hexadecimal representation of "]" can also be used to end a range. Ranges operate in the collating sequence of character values. They can also be used for characters specified numerically, for example [\000-\037]. Ranges can include any characters that are valid for the current mode. If a range that includes letters is used when caseless matching is set, it matches the letters in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character tables for a French locale are in use, [\xc8-\xcb] matches accented E characters in both cases. In UTF modes, PCRE supports the concept of case for characters with values greater than 128 only when it is compiled with Unicode property support. The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W may appear in a character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any hexadecimal digit. In UTF modes, the ucp option affects the meanings of \d, \s, \w and their upper case partners, just as it does when they appear outside a character class, as described in the section entitled "Generic character types" above. The escape sequence \b has a different meaning inside a character class; it matches the backspace character. The sequences \B, \N, \R, and \X are not special inside a character class. Like any other unrecognized escape sequences, they are treated as the literal characters "B", "N", "R", and "X". A circumflex can conveniently be used with the upper case character types to specify a more restricted set of characters than the matching lower case type. For example, the class [^\W_] matches any letter or digit, but not underscore, whereas [\w] includes underscore. A positive character class should be read as "something OR something OR ..." and a negative class as "NOT something AND NOT something AND NOT ...". The only metacharacters that are recognized in character classes are backslash, hyphen (only where it can be interpreted as specifying a range), circumflex (only at the start), opening square bracket (only when it can be interpreted as introducing a POSIX class name - see the next section), and the terminating closing square bracket. However, escaping other non-alphanumeric characters does no harm.POSIX CHARACTER CLASSES¶
Perl supports the POSIX notation for character classes. This uses names enclosed by [: and :] within the enclosing square brackets. PCRE also supports this notation. For example, [01[:alpha:]%] matches "0", "1", any alphabetic character, or "%". The supported class names are:- alnum:
- letters and digits
- alpha:
- letters
- ascii:
- character codes 0 - 127
- blank:
- space or tab only
- cntrl:
- control characters
- digit:
- decimal digits (same as \d)
- graph:
- printing characters, excluding space
- lower:
- lower case letters
- print:
- printing characters, including space
- punct:
- printing characters, excluding letters and digits and space
- space:
- whitespace (not quite the same as \s)
- upper:
- upper case letters
- word:
- "word" characters (same as \w)
- xdigit:
- hexadecimal digits
- [:alnum:]:
- becomes \p{Xan}
- [:alpha:]:
- becomes \p{L}
- [:blank:]:
- becomes \h
- [:digit::
- becomes \p{Nd}
- [:lower:]:
- becomes \p{Ll}
- [:space:]:
- becomes \p{Xps}
- [:upper::
- becomes \p{Lu}
- [:word:]:
- becomes \p{Xwd}
VERTICAL BAR¶
Vertical bar characters are used to separate alternative patterns. For example, the pattern gilbert|sullivan matches either "gilbert" or "sullivan". Any number of alternatives may appear, and an empty alternative is permitted (matching the empty string). The matching process tries each alternative in turn, from left to right, and the first one that succeeds is used. If the alternatives are within a subpattern (defined below), "succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern.INTERNAL OPTION SETTING¶
The settings of the caseless, multiline, dotall, and extended options (which are Perl-compatible) can be changed from within the pattern by a sequence of Perl option letters enclosed between "(?" and ")". The option letters are- i:
- for caseless
- m:
- for multiline
- s:
- for dotall
- x:
- for extended
SUBPATTERNS¶
Subpatterns are delimited by parentheses (round brackets), which can be nested. Turning part of a pattern into a subpattern does two things: 1. It localizes a set of alternatives. For example, the pattern cat(aract|erpillar|) matches "cataract", "caterpillar", or "cat". Without the parentheses, it would match "cataract", "erpillar" or an empty string. 2. It sets up the subpattern as a capturing subpattern. This means that, when the complete pattern matches, that portion of the subject string that matched the subpattern is passed back to the caller via the return value of re:run/3. Opening parentheses are counted from left to right (starting from 1) to obtain numbers for the capturing subpatterns.For example, if the string "the red king" is matched against the pattern the ((red|white) (king|queen)) the captured substrings are "red king", "red", and "king", and are numbered 1, 2, and 3, respectively. The fact that plain parentheses fulfil two functions is not always helpful. There are often times when a grouping subpattern is required without a capturing requirement. If an opening parenthesis is followed by a question mark and a colon, the subpattern does not do any capturing, and is not counted when computing the number of any subsequent capturing subpatterns. For example, if the string "the white queen" is matched against the pattern the ((?:red|white) (king|queen)) the captured substrings are "white queen" and "queen", and are numbered 1 and 2. The maximum number of capturing subpatterns is 65535. As a convenient shorthand, if any option settings are required at the start of a non-capturing subpattern, the option letters may appear between the "?" and the ":". Thus the two patterns- *
- (?i:saturday|sunday)
- *
- (?:(?i)saturday|sunday)
DUPLICATE SUBPATTERN NUMBERS¶
Perl 5.10 introduced a feature whereby each alternative in a subpattern uses the same numbers for its capturing parentheses. Such a subpattern starts with (?| and is itself a non-capturing subpattern. For example, consider this pattern: (?|(Sat)ur|(Sun))day Because the two alternatives are inside a (?| group, both sets of capturing parentheses are numbered one. Thus, when the pattern matches, you can look at captured substring number one, whichever alternative matched. This construct is useful when you want to capture part, but not all, of one of a number of alternatives. Inside a (?| group, parentheses are numbered as usual, but the number is reset at the start of each branch. The numbers of any capturing parentheses that follow the subpattern start after the highest number used in any branch. The following example is taken from the Perl documentation. The numbers underneath show in which buffer the captured content will be stored.# before ---------------branch-reset----------- after / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x # 1 2 2 3 2 3 4A back reference to a numbered subpattern uses the most recent value that is set for that number by any subpattern. The following pattern matches "abcabc" or "defdef": /(?|(abc)|(def))\1/ In contrast, a subroutine call to a numbered subpattern always refers to the first one in the pattern with the given number. The following pattern matches "abcabc" or "defabc": /(?|(abc)|(def))(?1)/ If a condition test for a subpattern's having matched refers to a non-unique number, the test is true if any of the subpatterns of that number have matched. An alternative approach to using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section.
NAMED SUBPATTERNS¶
Identifying capturing parentheses by number is simple, but it can be very hard to keep track of the numbers in complicated regular expressions. Furthermore, if an expression is modified, the numbers may change. To help with this difficulty, PCRE supports the naming of subpatterns. This feature was not added to Perl until release 5.10. Python had the feature earlier, and PCRE introduced it at release 4.0, using the Python syntax. PCRE now supports both the Perl and the Python syntax. Perl allows identically numbered subpatterns to have different names, but PCRE does not. In PCRE, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing parentheses from other parts of the pattern, such as back references, recursion, and conditions, can be made by name as well as by number. Names consist of up to 32 alphanumeric characters and underscores. Named capturing parentheses are still allocated numbers as well as names, exactly as if the names were not present. The capture specification to re:run/3 can use named values if they are present in the regular expression. By default, a name must be unique within a pattern, but it is possible to relax this constraint by setting the dupnames option at compile time. (Duplicate names are also always permitted for subpatterns with the same number, set up as described in the previous section.) Duplicate names can be useful for patterns where only one instance of the named parentheses can match. Suppose you want to match the name of a weekday, either as a 3-letter abbreviation or as the full name, and in both cases you want to extract the abbreviation. This pattern (ignoring the line breaks) does the job:(?<DN>Mon|Fri|Sun)(?:day)?| (?<DN>Tue)(?:sday)?| (?<DN>Wed)(?:nesday)?| (?<DN>Thu)(?:rsday)?| (?<DN>Sat)(?:urday)?There are five capturing substrings, but only one is ever set after a match. (An alternative way of solving this problem is to use a "branch reset" subpattern, as described in the previous section.) In case of capturing named subpatterns which names are not unique, the first matching occurrence (counted from left to right in the subject) is returned from re:exec/3, if the name is specified in the values part of the capture statement. The all_names capturing value will match all of the names in the same way. Warning: You cannot use different names to distinguish between two subpatterns with the same number because PCRE uses only the numbers when matching. For this reason, an error is given at compile time if different names are given to subpatterns with the same number. However, you can give the same name to subpatterns with the same number, even when dupnames is not set.
REPETITION¶
Repetition is specified by quantifiers, which can follow any of the following items:- *
- a literal data character
- *
- the dot metacharacter
- *
- the \C escape sequence
- *
- the \X escape sequence
- *
- the \R escape sequence
- *
- an escape such as \d or \pL that matches a single character
- *
- a character class
- *
- a back reference (see next section)
- *
- a parenthesized subpattern (including assertions)
- *
- a subroutine call to a subpattern (recursive or otherwise)
- *:
- is equivalent to {0,}
- +:
- is equivalent to {1,}
- ?:
- is equivalent to {0,1}
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS¶
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of what follows normally causes the repeated item to be re-evaluated to see if a different number of repeats allows the rest of the pattern to match. Sometimes it is useful to prevent this, either to change the nature of the match, or to cause it fail earlier than it otherwise might, when the author of the pattern knows there is no point in carrying on. Consider, for example, the pattern \d+foo when applied to the subject line 123456bar After matching all 6 digits and then failing to match "foo", the normal action of the matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and so on, before ultimately failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides the means for specifying that once a subpattern has matched, it is not to be re-evaluated in this way. If we use atomic grouping for the previous example, the matcher gives up immediately on failing to match "foo" the first time. The notation is a kind of special parenthesis, starting with (?> as in this example: (?>\d+)foo This kind of parenthesis "locks up" the part of the pattern it contains once it has matched, and a failure further into the pattern is prevented from backtracking into it. Backtracking past it to previous items, however, works as normal. An alternative description is that a subpattern of this type matches the string of characters that an identical standalone pattern would match, if anchored at the current point in the subject string. Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as the above example can be thought of as a maximizing repeat that must swallow everything it can. So, while both \d+ and \d+? are prepared to adjust the number of digits they match in order to make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits. Atomic groups in general can of course contain arbitrarily complicated subpatterns, and can be nested. However, when the subpattern for an atomic group is just a single repeated item, as in the example above, a simpler notation, called a "possessive quantifier" can be used. This consists of an additional + character following a quantifier. Using this notation, the previous example can be rewritten as \d++foo Note that a possessive quantifier can be used with an entire group, for example: (abc|xyz){2,3}+ Possessive quantifiers are always greedy; the setting of the ungreedy option is ignored. They are a convenient notation for the simpler forms of atomic group. However, there is no difference in the meaning of a possessive quantifier and the equivalent atomic group, though there may be a performance difference; possessive quantifiers should be slightly faster. The possessive quantifier syntax is an extension to the Perl 5.8 syntax. Jeffrey Friedl originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he built Sun's Java package, and PCRE copied it from there. It ultimately found its way into Perl at release 5.10. PCRE has an optimization that automatically "possessifies" certain simple pattern constructs. For example, the sequence A+B is treated as A++B because there is no point in backtracking into a sequence of A's when B must follow. When a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited number of times, the use of an atomic group is the only way to avoid some failing matches taking a very long time indeed. The pattern (\D+|<\d+>)*[!?] matches an unlimited number of substrings that either consist of non-digits, or digits enclosed in <>, followed by either ! or ?. When it matches, it runs quickly. However, if it is applied to aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa it takes a long time before reporting failure. This is because the string can be divided between the internal \D+ repeat and the external * repeat in a large number of ways, and all have to be tried. (The example uses [!?] rather than a single character at the end, because both PCRE and Perl have an optimization that allows for fast failure when a single character is used. They remember the last single character that is required for a match, and fail early if it is not present in the string.) If the pattern is changed so that it uses an atomic group, like this: ((?>\D+)|<\d+>)*[!?] sequences of non-digits cannot be broken, and failure happens quickly.BACK REFERENCES¶
Outside a character class, a backslash followed by a digit greater than 0 (and possibly further digits) is a back reference to a capturing subpattern earlier (that is, to its left) in the pattern, provided there have been that many previous capturing left parentheses. However, if the decimal number following the backslash is less than 10, it is always taken as a back reference, and causes an error only if there are not that many capturing left parentheses in the entire pattern. In other words, the parentheses that are referenced need not be to the left of the reference for numbers less than 10. A "forward back reference" of this type can make sense when a repetition is involved and the subpattern to the right has participated in an earlier iteration. It is not possible to have a numerical "forward back reference" to a subpattern whose number is 10 or more using this syntax because a sequence such as \50 is interpreted as a character defined in octal. See the subsection entitled "Non-printing characters" above for further details of the handling of digits following a backslash. There is no such problem when named parentheses are used. A back reference to any subpattern is possible using named parentheses (see below). Another way of avoiding the ambiguity inherent in the use of digits following a backslash is to use the \g escape sequence. This escape must be followed by an unsigned number or a negative number, optionally enclosed in braces. These examples are all identical:- *
- (ring), \1
- *
- (ring), \g1
- *
- (ring), \g{1}
- *
- (?<p1>(?i)rah)\s+\k<p1>
- *
- (?'p1'(?i)rah)\s+\k{p1}
- *
- (?P<p1>(?i)rah)\s+(?P=p1)
- *
- (?<p1>(?i)rah)\s+\g{p1}
ASSERTIONS¶
An assertion is a test on the characters following or preceding the current matching point that does not actually consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above. More complicated assertions are coded as subpatterns. There are two kinds: those that look ahead of the current position in the subject string, and those that look behind it. An assertion subpattern is matched in the normal way, except that it does not cause the current matching position to be changed. Assertion subpatterns are not capturing subpatterns. If such an assertion contains capturing subpatterns within it, these are counted for the purposes of numbering the capturing subpatterns in the whole pattern. However, substring capturing is carried out only for positive assertions. (Perl sometimes, but not always, does do capturing in negative assertions.) For compatibility with Perl, assertion subpatterns may be repeated; though it makes no sense to assert the same thing several times, the side effect of capturing parentheses may occasionally be useful. In practice, there only three cases:- (1):
- If the quantifier is {0}, the assertion is never obeyed during matching. However, it may contain internal capturing parenthesized groups that are called from elsewhere via the subroutine mechanism.
- (2):
- If quantifier is {0,n} where n is greater than zero, it is treated as if it were {0,1}. At run time, the rest of the pattern match is tried with and without the assertion, the order depending on the greediness of the quantifier.
- (3):
- If the minimum repetition is greater than zero, the quantifier is ignored. The assertion is obeyed just once when encountered during matching.
CONDITIONAL SUBPATTERNS¶
It is possible to cause the matching process to obey a subpattern conditionally or to choose between two alternative subpatterns, depending on the result of an assertion, or whether a specific capturing subpattern has already been matched. The two possible forms of conditional subpattern are:- *
- (?(condition)yes-pattern)
- *
- (?(condition)yes-pattern|no-pattern)
(?(?=[^a-z]*[a-z]) \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed by a letter. In other words, it tests for the presence of at least one letter in the subject. If a letter is found, the subject is matched against the first alternative; otherwise it is matched against the second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
COMMENTS¶
There are two ways of including comments in patterns that are processed by PCRE. In both cases, the start of the comment must not be in a character class, nor in the middle of any other sequence of related characters such as (?: or a subpattern name or number. The characters that make up a comment play no part in the pattern matching. The sequence (?# marks the start of a comment that continues up to the next closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED option is set, an unescaped # character also introduces a comment, which in this case continues to immediately after the next newline character or character sequence in the pattern. Which characters are interpreted as newlines is controlled by the options passed to a compiling function or by a special sequence at the start of the pattern, as described in the section entitled "Newline conventions" above. Note that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count. For example, consider this pattern when extended is set, and the default newline convention is in force: abc #comment \n still comment On encountering the # character, pcre_compile() skips along, looking for a newline in the pattern. The sequence \n is still literal at this stage, so it does not terminate the comment. Only an actual character with the code value 0x0a (the default newline) does so.RECURSIVE PATTERNS¶
Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses. Without the use of recursion, the best that can be done is to use a pattern that matches up to some fixed depth of nesting. It is not possible to handle an arbitrary nesting depth. For some time, Perl has provided a facility that allows regular expressions to recurse (amongst other things). It does this by interpolating Perl code in the expression at run time, and the code can refer to the expression itself. A Perl pattern using code interpolation to solve the parentheses problem can be created like this: $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively to the pattern in which it appears. Obviously, PCRE cannot support the interpolation of Perl code. Instead, it supports special syntax for recursion of the entire pattern, and also for individual subpattern recursion. After its introduction in PCRE and Python, this kind of recursion was subsequently introduced into Perl at release 5.10. A special item that consists of (? followed by a number greater than zero and a closing parenthesis is a recursive subroutine call of the subpattern of the given number, provided that it occurs inside that subpattern. (If not, it is a non-recursive subroutine call, which is described in the next section.) The special item (?R) or (?0) is a recursive call of the entire regular expression. This PCRE pattern solves the nested parentheses problem (assume the extended option is set so that whitespace is ignored): \( ( [^()]++ | (?R) )* \) First it matches an opening parenthesis. Then it matches any number of substrings which can either be a sequence of non-parentheses, or a recursive match of the pattern itself (that is, a correctly parenthesized substring). Finally there is a closing parenthesis. Note the use of a possessive quantifier to avoid backtracking into sequences of non-parentheses. If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you could use this: ( \( ( [^()]++ | (?1) )* \) ) We have put the pattern into parentheses, and caused the recursion to refer to them instead of the whole pattern. In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made easier by the use of relative references. Instead of (?1) in the pattern above you can write (?-2) to refer to the second most recently opened parentheses preceding the recursion. In other words, a negative number counts capturing parentheses leftwards from the point at which it is encountered. It is also possible to refer to subsequently opened parentheses, by writing references such as (?+2). However, these cannot be recursive because the reference is not inside the parentheses that are referenced. They are always non-recursive subroutine calls, as described in the next section. An alternative approach is to use named parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We could rewrite the above example as follows: (?<pn> \( ( [^()]++ | (?&pn) )* \) ) If there is more than one subpattern with the same name, the earliest one is used. This particular example pattern that we have been looking at contains nested unlimited repeats, and so the use of a possessive quantifier for matching strings of non-parentheses is important when applying the pattern to strings that do not match. For example, when this pattern is applied to (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() it yields "no match" quickly. However, if a possessive quantifier is not used, the match runs for a very long time indeed because there are so many different ways the + and * repeats can carve up the subject, and all have to be tested before failure can be reported. At the end of a match, the values of capturing parentheses are those from the outermost level. If the pattern above is matched against (ab(cd)ef) the value for the inner capturing parentheses (numbered 2) is "ef", which is the last value taken on at the top level. If a capturing subpattern is not matched at the top level, its final captured value is unset, even if it was (temporarily) set at a deeper level during the matching process. Do not confuse the (?R) item with the condition (R), which tests for recursion. Consider this pattern, which matches text in angle brackets, allowing for arbitrary nesting. Only digits are allowed in nested brackets (that is, when recursing), whereas any characters are permitted at the outer level. < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > In this pattern, (?(R) is the start of a conditional subpattern, with two different alternatives for the recursive and non-recursive cases. The (?R) item is the actual recursive call. Differences in recursion processing between PCRE and Perl Recursion processing in PCRE differs from Perl in two important ways. In PCRE (like Python, but unlike Perl), a recursive subpattern call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure. This can be illustrated by the following pattern, which purports to match a palindromic string that contains an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"): ^(.|(.)(?1)\2)$ The idea is that it either matches a single character, or two identical characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE it does not if the pattern is longer than three characters. Consider the subject string "abcba": At the top level, the first character is matched, but as it is not at the end of the string, the first alternative fails; the second alternative is taken and the recursion kicks in. The recursive call to subpattern 1 successfully matches the next character ("b"). (Note that the beginning and end of line tests are not part of the recursion). Back at the top level, the next character ("c") is compared with what subpattern 2 matched, which was "a". This fails. Because the recursion is treated as an atomic group, there are now no backtracking points, and so the entire match fails. (Perl is able, at this point, to re-enter the recursion and try the second alternative.) However, if the pattern is written with the alternatives in the other order, things are different: ^((.)(?1)\2|.)$ This time, the recursing alternative is tried first, and continues to recurse until it runs out of characters, at which point the recursion fails. But this time we do have another alternative to try at the higher level. That is the big difference: in the previous case the remaining alternative is at a deeper recursion level, which PCRE cannot use. To change the pattern so that it matches all palindromic strings, not just those with an odd number of characters, it is tempting to change the pattern to this: ^((.)(?1)\2|.?)$ Again, this works in Perl, but not in PCRE, and for the same reason. When a deeper recursion has matched a single character, it cannot be entered again in order to match an empty string. The solution is to separate the two cases, and write out the odd and even cases as alternatives at the higher level: ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) If you want to match typical palindromic phrases, the pattern has to ignore all non-word characters, which can be done like this: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ If run with the caseless option, this pattern matches phrases such as "A man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note the use of the possessive quantifier *+ to avoid backtracking into sequences of non-word characters. Without this, PCRE takes a great deal longer (ten times or more) to match typical phrases, and Perl takes so long that you think it has gone into a loop. WARNING: The palindrome-matching patterns above work only if the subject string does not start with a palindrome that is shorter than the entire string. For example, although "abcba" is correctly matched, if the subject is "ababa", PCRE finds the palindrome "aba" at the start, then fails at top level because the end of the string does not follow. Once again, it cannot jump back into the recursion to try other alternatives, so the entire match fails. The second way in which PCRE and Perl differ in their recursion processing is in the handling of captured values. In Perl, when a subpattern is called recursively or as a subpattern (see the next section), it has no access to any values that were captured outside the recursion, whereas in PCRE these values can be referenced. Consider this pattern: ^(.)(\1|a(?2)) In PCRE, this pattern matches "bab". The first capturing parentheses match "b", then in the second group, when the back reference \1 fails to match "b", the second alternative matches "a" and then recurses. In the recursion, \1 does now match "b" and so the whole match succeeds. In Perl, the pattern fails to match because inside the recursive call \1 cannot access the externally set value.SUBPATTERNS AS SUBROUTINES¶
If the syntax for a recursive subpattern call (either by number or by name) is used outside the parentheses to which it refers, it operates like a subroutine in a programming language. The called subpattern may be defined before or after the reference. A numbered reference can be absolute or relative, as in these examples:- *
- (...(absolute)...)...(?2)...
- *
- (...(relative)...)...(?-1)...
- *
- (...(?+1)...(relative)...
ONIGURUMA SUBROUTINE SYNTAX¶
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a subroutine, possibly recursively. Here are two of the examples used above, rewritten using this syntax: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) (sens|respons)e and \g'1'ibility PCRE supports an extension to Oniguruma: if a number is preceded by a plus or a minus sign it is taken as a relative reference. For example: (abc)(?i:\g<-1>) Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call.BACKTRACKING CONTROL¶
Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are still described in the Perl documentation as "experimental and subject to change or removal in a future version of Perl". It goes on to say: "Their usage in production code should be noted to avoid problems during upgrades." The same remarks apply to the PCRE features described in this section. The new verbs make use of what was previously invalid syntax: an opening parenthesis followed by an asterisk. They are generally of the form (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving differently depending on whether or not a name is present. A name is any sequence of characters that does not include a closing parenthesis. The maximum length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit libraries. If the name is empty, that is, if the closing parenthesis immediately follows the colon, the effect is as if the colon were not there. Any number of these verbs may occur in a pattern. The behaviour of these verbs in repeated groups, assertions, and in subpatterns called as subroutines (whether or not recursively) is documented below. Optimizations that affect backtracking verbs PCRE contains some optimizations that are used to speed up matching by running some checks at the start of each match attempt. For example, it may know the minimum length of matching subject, or that a particular character must be present. When one of these optimizations bypasses the running of a match, any included backtracking verbs will not, of course, be processed. You can suppress the start-of-match optimizations by setting the no_start_optimize option when calling re:compile/2 or re:run/3, or by starting the pattern with (*NO_START_OPT). Experiments with Perl suggest that it too has similar optimizations, sometimes leading to anomalous results. Verbs that act immediately The following verbs act as soon as they are encountered. They may not be followed by a name. (*ACCEPT) This verb causes the match to end successfully, skipping the remainder of the pattern. However, when it is inside a subpattern that is called as a subroutine, only that subpattern is ended successfully. Matching then continues at the outer level. If (*ACCEPT) in triggered in a positive assertion, the assertion succeeds; in a negative assertion, the assertion fails. If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For example: A((?:A|B(*ACCEPT)|C)D) This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by the outer parentheses. (*FAIL) or (*F) This verb causes a matching failure, forcing backtracking to occur. It is equivalent to (?!) but easier to read. The Perl documentation notes that it is probably useful only when combined with (?{}) or (??{}). Those are, of course, Perl features that are not present in PCRE. The nearest equivalent is the callout feature, as for example in this pattern: a+(?C)(*FAIL) A match with the string "aaaa" always fails, but the callout is taken before each backtrack happens (in this example, 10 times). Recording which path was taken There is one verb whose main purpose is to track how a match was arrived at, though it also has a secondary use in conjunction with advancing the match starting point (see (*SKIP) below).Warning:
In Erlang, there is no interface to retrieve a mark with re:run/{2,3], so
only the secondary purpose is relevant to the Erlang programmer!
The rest of this section is therefore deliberately not adapted for reading by
the Erlang programmer, however the examples might help in understanding NAMES
as they can be used by (*SKIP).
(*MARK:NAME) or (*:NAME)
A name is always required with this verb. There may be as many instances of
(*MARK) as you like in a pattern, and their names do not have to be unique.
When a match succeeds, the name of the last-encountered (*MARK:NAME),
(*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
caller as described in the section entitled "Extra data for
pcre_exec()" in the pcreapi documentation. Here is an
example of pcretest output, where the /K modifier requests the
retrieval and outputting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K data> XY 0: XY MK: A XZ 0: XZ MK: BThe (*MARK) name is tagged with "MK:" in this output, and in this example it indicates which of the two alternatives matched. This is a more efficient way of obtaining this information than putting each alternative in its own capturing parentheses. If a verb with a name is encountered in a positive assertion that is true, the name is recorded and passed back if it is the last-encountered. This does not happen for negative assertions or failing positive assertions. After a partial match or a failed match, the last encountered name in the entire match process is returned. For example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K data> XP No match, mark = BNote that in this unanchored example the mark is retained from the match attempt that started at the letter "X" in the subject. Subsequent match attempts starting at "P" and then with an empty string do not get as far as the (*MARK) item, but nevertheless do not reset it. Verbs that act after backtracking The following verbs do nothing when they are encountered. Matching continues with what follows, but if there is no subsequent match, causing a backtrack to the verb, a failure is forced. That is, backtracking cannot pass to the left of the verb. However, when one of these verbs appears inside an atomic group or an assertion that is true, its effect is confined to that group, because once the group has been matched, there is never any backtracking into it. In this situation, backtracking can "jump back" to the left of the entire atomic group or assertion. (Remember also, as stated above, that this localization also applies in subroutine calls.) These verbs differ in exactly what kind of failure occurs when backtracking reaches them. The behaviour described below is what happens when the verb is not in a subroutine or an assertion. Subsequent sections cover these special cases. (*COMMIT) This verb, which may not be followed by a name, causes the whole match to fail outright if there is a later matching failure that causes backtracking to reach it. Even if the pattern is unanchored, no further attempts to find a match by advancing the starting point take place. If (*COMMIT) is the only backtracking verb that is encountered, once it has been passed re:run/{2,3} is committed to finding a match at the current starting point, or not at all. For example: a+(*COMMIT)b This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor, or "I've started, so I must finish." The name of the most recently passed (*MARK) in the path is passed back when (*COMMIT) forces a match failure. If there is more than one backtracking verb in a pattern, a different one that follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a match does not always guarantee that a match must be at this starting point. Note that (*COMMIT) at the start of a pattern is not the same as an anchor, unless PCRE's start-of-match optimizations are turned off, as shown in this example:
1> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list}]). {match,["abc"]} 2> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list},no_start_optimize]). nomatchPCRE knows that any match must start with "a", so the optimization skips along the subject to "a" before running the first match attempt, which succeeds. When the optimization is disabled by the no_start_optimize option, the match starts at "x" and so the (*COMMIT) causes it to fail without trying any other starting points. (*PRUNE) or (*PRUNE:NAME) This verb causes the match to fail at the current starting position in the subject if there is a later matching failure that causes backtracking to reach it. If the pattern is unanchored, the normal "bumpalong" advance to the next starting character then happens. Backtracking can occur as usual to the left of (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but if there is no match to the right, backtracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative to an atomic group or possessive quantifier, but there are some uses of (*PRUNE) that cannot be expressed in any other way. In an anchored pattern (*PRUNE) has the same effect as (*COMMIT). The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is remembered for passing back to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
Warning:
The fact that (*PRUNE:NAME) remembers the name is useless to the Erlang
programmer, as names can not be retrieved.
(*SKIP)
This verb, when given without a name, is like (*PRUNE), except that if the
pattern is unanchored, the "bumpalong" advance is not to the next
character, but to the position in the subject where (*SKIP) was encountered.
(*SKIP) signifies that whatever text was matched leading up to it cannot be
part of a successful match. Consider:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt fails
(starting at the first character in the string), the starting point skips on
to start the next attempt at "c". Note that a possessive quantifer
does not have the same effect as this example; although it would suppress
backtracking during the first match attempt, the second attempt would start at
the second character instead of skipping on to "c".
(*SKIP:NAME)
When (*SKIP) has an associated name, its behaviour is modified. When it is
triggered, the previous path through the pattern is searched for the most
recent (*MARK) that has the same name. If one is found, the
"bumpalong" advance is to the subject position that corresponds to
that (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
matching name is found, the (*SKIP) is ignored.
Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
names that are set by (*PRUNE:NAME) or (*THEN:NAME).
(*THEN) or (*THEN:NAME)
This verb causes a skip to the next innermost alternative when backtracking
reaches it. That is, it cancels any further backtracking within the current
alternative. Its name comes from the observation that it can be used for a
pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
If the COND1 pattern matches, FOO is tried (and possibly further items after the
end of the group if FOO succeeds); on failure, the matcher skips to the second
alternative and tries COND2, without backtracking into COND1. If that succeeds
and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no more
alternatives, so there is a backtrack to whatever came before the entire
group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). It is
like (*MARK:NAME) in that the name is remembered for passing back to the
caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
Warning:
The fact that (*THEN:NAME) remembers the name is useless to the Erlang
programmer, as names can not be retrieved.
A subpattern that does not contain a | character is just a part of the enclosing
alternative; it is not a nested alternation with only one alternative. The
effect of (*THEN) extends beyond such a subpattern to the enclosing
alternative. Consider this pattern, where A, B, etc. are complex pattern
fragments that do not contain any | characters at this level:
A (B(*THEN)C) | D
If A and B are matched, but there is a failure in C, matching does not backtrack
into A; instead it moves to the next alternative, that is, D. However, if the
subpattern containing (*THEN) is given an alternative, it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner subpattern. After a failure
in C, matching moves to (*FAIL), which causes the whole subpattern to fail
because there are no more alternatives to try. In this case, matching does now
backtrack into A.
Note that a conditional subpattern is not considered as having two alternatives,
because only one is ever used. In other words, the | character in a
conditional subpattern has a different meaning. Ignoring white space,
consider:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. Because .*? is
ungreedy, it initially matches zero characters. The condition (?=a) then
fails, the character "b" is matched, but "c" is not. At
this point, matching does not backtrack to .*? as might perhaps be expected
from the presence of the | character. The conditional subpattern is part of
the single alternative that comprises the whole pattern, and so the match
fails. (If there was a backtrack into .*?, allowing it to match "b",
the match would succeed.)
The verbs just described provide four different "strengths" of control
when subsequent matching fails. (*THEN) is the weakest, carrying on the match
at the next alternative. (*PRUNE) comes next, failing the match at the current
starting position, but allowing an advance to the next character (for an
unanchored pattern). (*SKIP) is similar, except that the advance may be more
than one character. (*COMMIT) is the strongest, causing the entire match to
fail.
More than one backtracking verb
If more than one backtracking verb is present in a pattern, the one that is
backtracked onto first acts. For example, consider this pattern, where A, B,
etc. are complex pattern fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
the next alternative (ABD) to be tried. This behaviour is consistent, but is
not always the same as Perl's. It means that if two or more backtracking verbs
appear in succession, all the the last of them has no effect. Consider this
example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto (*PRUNE) cases it
to be triggered, and its action is taken. There can never be a backtrack onto
(*COMMIT).
Backtracking verbs in repeated groups
PCRE differs from Perl in its handling of backtracking verbs in repeated groups.
For example, consider:
/(a(*COMMIT)b)+ac/
If the subject is "abac", Perl matches, but PCRE fails because the
(*COMMIT) in the second repeat of the group acts.
Backtracking verbs in assertions
(*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
(*ACCEPT) in a positive assertion causes the assertion to succeed without any
further processing. In a negative assertion, (*ACCEPT) causes the assertion to
fail without any further processing.
The other backtracking verbs are not treated specially if they appear in a
positive assertion. In particular, (*THEN) skips to the next alternative in
the innermost enclosing group that has alternations, whether or not this is
within the assertion.
Negative assertions are, however, different, in order to ensure that changing a
positive assertion into a negative assertion changes its result. Backtracking
into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
without considering any further alternative branches in the assertion.
Backtracking into (*THEN) causes it to skip to the next enclosing alternative
within the assertion (the normal behaviour), but if the assertion does not
have such an alternative, (*THEN) behaves like (*PRUNE).
Backtracking verbs in subroutines
These behaviours occur whether or not the subpattern is called recursively.
Perl's treatment of subroutines is different in some cases.
(*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
an immediate backtrack.
(*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
succeed without any further processing. Matching then continues after the
subroutine call.
(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
the subroutine match to fail.
(*THEN) skips to the next alternative in the innermost enclosing group within
the subpattern that has alternatives. If there is no such group within the
subpattern, (*THEN) causes the subroutine match to fail.stdlib 2.2 | Ericsson AB |