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flexc++input(7) flexc++ input file organization flexc++input(7)


flexc++input - Organization of flexc++’s input s


Flexc++(1) was designed after flex(1) and flex++(1). Like these two programs flexc++ generates code performing pattern-matching on text, possibly executing actions when certain regular expressions are recognized.

Refer to flexc++(1) for a general overview. This manual page describes how flexc++’s input s should be organized. It contains the following sections:

1. SPECIFICATION FILE(S): the format and contents of flexc++ input files, specifying the Scanner’s characteristics
2. FILE SWITCHING: how to switch to another input specification file
3. DIRECTIVES: directives that can be used in input specification files
4. MINI SCANNERS: how to declare mini-scanners
5. DEFINITIONS: how to define symbolic names for regular expressions
6. %% SEPARATOR: the separator between the input specification sections
7. REGULAR EXPRESSIONS: regular expressions supported by flexc++
8. SPECIFICATION EXAMPLE: an example of a specification file


Starting with version 2.07.00 flexc++ reserved identifiers no longer end in two underscore characters, but in one. This modification was necessary because according to the C++ standard identifiers having two or more consecutive underscore characters are reserved by the language. In practice this could require some minor modifications of existing source files using flexc++’s facilities, most likely limited to changing StartCondition__ into StartCondition_ and changing PostEnum__ into PostEnum_.

The complete list of affected names is:

ActionType_, Leave_, StartConditon_, PostEnum_;
actionType_, continue_, echoCh_, echoFirst_, executeAction_, getRange_, get_, istreamName_, lex_, lop1_, lop2_, lop3_, lop4_, lopf_, matched_, noReturn_, print_, pushFront_, reset_, return_;
d_in_ d_token_ s_finIdx_, s_interactive_, s_maxSizeofStreamStack_, s_nRules_, s_rangeOfEOF_, s_ranges_, s_rf_.


Flexc++ expects an input file containing directives and the regular expressions that should be recognized by objects of the scanner class generated by flexc++. In this man page the elements and organization of flexc++’s input file is described.

Flexc++’s input file consists of two sections, separated from each other by a line merely containing two consecutive percent characters:


The section before this separator contains directives; the section following this separator contains regular expressions and possibly actions to perform when these regular expressions are matched by the object of the scanner class generated by flexc++. If a second line is encountered immediately beginning with two consecutive percent characters then this ends flexc++’s input file processing. See also section 6 (%% SEPARATOR) below.

White space is usually ignored, as is comment, which may be of the traditional C form (i.e., /*, followed by (possibly multi-line) comment text, followed by */, and it may be C++ end-of-line comment: two consecutive slashes (//) start the comment, which continues up to the next newline character.


Flexc++’s input file may be split into multiple files. This allows for the definition of logically separate elements of the specifications in different files. Include directives must be specified on separate lines and may not contain any other information than the (path)name of the file to switch to. File names may be surrounded by double quotes, but these double quotes are optional and are ignored (removed) when encountered. All remaining characters define the name of the subsequently processed file by flexc++. White space characters following //include and preceding the end of the line are ignored. To switch files the following stanza is used:

//include file-location

The //include directive must start in the line’s first column. File locations can be absolute or relative to the location of the file containing the //include directive. Once flexc++ has switched to another file that file’s directory becomes flexc++’s working directory.

Once the end of file of a file has been reached, processing continues at the line beyond the //include directive of the previously scanned file, and flexc++’s working directory is reset to the working directory of the file to which flexc++ returns. The end-of-file of the file that was initially specified when flexc++ started indicates the end of flexc++’s rules specification.


The first section of flexc++’s input file consists of directives. In addition it may associate regular expressions with symbolic names, allowing you to use these identifiers in the rules section. Each directive is defined on a line of its own. When available, directives are overridden by flexc++ command line options.

Some directives require arguments, which are usually provided following separating (but optional) = characters. Arguments of directives are text, surrounded by double quotes (strings), or embedded in raw string literals (rawstrings). Double quotes or backslashes inside strings must themselves be preceded by backslashes; these backslashes are not required when rawstrings are used.

The %s and %x directives are immediately followed by name lists, consisting of identifiers separated by blanks. Here is an example of the definition of a directive:

%class-name = "MyScanner"

Directives accepting a `filename’ do not accept path names, i.e., they cannot contain directory separators (/); options accepting a ’pathname’ may contain directory separators. A ’pathname’ using blank characters should be surrounded by double quotes.

Some directives may generate errors. This happens when a directive conflicts with the contents of an existing file which flexc++ cannot modify (e.g., a scanner class header file exists, but doesn’t define a name space, but a %namespace directive was provided). To solve the error the offending directive could be omitted, the existing file could be removed, or the existing file could be hand-edited according to the directive’s specification. Note that flexc++ currently does not handle the opposite error condition: if a previously used directive is omitted, then flexc++ does not detect the inconsistency. In those cases you may encounter compilation errors.

%baseclass-header = "filename"
Defines the name of the file to contain the scanner class’s base class interface. Corresponding command-line option: --baseclass-header.
It is an error if this directive is used and an already existing scanner-class header file does not include `filename’.
Generates a scanner which case insensitively matches regular expressions. All regular expressions specified in flexc++’s input file are interpreted case insensitively and the resulting scanner object will case insensitively interpret its input.
Corresponding command-line option: --cases-insensitive.
When this directive is specified the resulting scanner does not distinguish between the following rules:

First // initial F is transformed to f
FIRST // all capitals are transformed to lower case chars
With a case-insensitive scanner only the first rule can be matched, and flexc++ will issue warnings for the second and third rule about rules that cannot be matched.
Input processed by a case-insensitive scanner is also handled case insensitively. The above mentioned First rule is matched for all of the following input words: first First FIRST firST.
Although the matching process proceeds case insensitively, the matched text (as returned by the scanner’s matched() member) always contains the original, unmodified text. So, with the above input matched() returns, respectively first, First, FIRST and firST, while matching the rule First.
%class-header = "filename"
Defines the name of the file to contain the scanner class’s interface. Corresponding command-line option: --class-header.
%class-name = "className"
Declares the name of the scanner class generated by flexc++. This directive corresponds to the %name directive used by flex++(1). Contrary to flex++’s %name declaration, class-name may appear anywhere in the first section of the grammar specification file. It may be defined only once. If no class-name is specified the default class name (Scanner) is used. Corresponding command-line option: --class-name.
It is an error if this directive is used and an already existing scanner-class header file does not define class `className’.
Provide lex and its support functions with debugging code, showing the actual parsing process on the standard output stream. When included, the debugging output is active by default, but its activity may be controlled using the setDebug(bool on-off) member. Note that no #ifdef DEBUG macros are used in the generated code.
%filenames = "basename"
Defines the basename of the Scanner.h, Scanner.ih, and Scannerbase.h files. E.g., when using the directive

%filenames = "scanner"
the names of the generated files are, respectively, scanner.h, scanner.ih, and scannerbase.h. Corresponding command-line option: --filenames. The name of the source file (by default is controlled by the %lex-source directive.
%implementation-header = "filename"
Defines the name of the file to contain the implementation header. Corresponding command-line option: --implementation-header.
It is an error if this directive is used and an already ’filename’ file does not include the scanner class header file.
%input-implementation = "sourcefile"
Defines the pathname of the file containing the implementation of a user-defined Input class.
%input-interface = "interface"
Defines the pathname of the file containing the interface of a user-defined Input class. See section 17. THE CLASS INPUT in the flexc++api(3) manual page for additional information about user-defined Input classes.
Generate an interactive scanner. An interactive scanner reads lines from the input stream, and then returns the tokens encountered on that line. The interactive scanner implemented by flexc++ only predefines the Scanner(std::istream &in, std::ostream &out) constructor, by default assuming that input is read from std::cin. See also section 1. INTERACTIVE SCANNER section in the flexc++api(3) manual page.
%lex-function-name = "funname"
Defines the name of the scanner class’s member to perform the lexical scanning. If this directive is omitted the default name (lex) is used. Corresponding command-line option: --lex-function-name.
%lex-source = "filename"
Defines the name of the file to contain the scanner member lex. Corresponding command-line option: --lex-source.
Do not put #line preprocessor directives in the file containing the scanner’s lex function. If omitted #line directives are added to this file, unless overridden by the command line options --lines and --no-lines.
%namespace = "identifer"
Define the scanner class in the namespace identifier. By default no namespace is used. If this directives is used the implementation header is provided with a commented out using namespace declaration for the requested namespace. In addition, the scanner and scanner base class header files also use the specified namespace to define their include guard directives.
It is an error if this directive is used and an already scanner-class header file does not define namespace identifier.
this directive results in the tokens as well as the matched text to be displayed on the standard output stream, just before returning the token to lex’s caller. Displaying is suppressed again when the file is generated without using this directive. The function showing the tokens (ScannerBase::print_) is called from Scanner::print(), which is defined in-line in Scanner.h. Calling ScannerBase::print_, therefore, can also easily be controlled by an option controlled by the program using the scanner object. this directive does not show the tokens returned and text matched by flexc++ itself when reading its input s. If that is what you want, use the --own-tokens option.
%s namelist
The %s directive is followed by a list of one or more identifiers, separated by blanks. Each identifier is the name of an inclusive start condition.
%skeleton-directory = "pathname"
Use pathname rather than the default (e.g., /usr/share/flexc++) path when looking for flexc++’s skeleton files. Corresponding command-line option: --skeleton-directory.
startcondition-name = "startconditionName"
By default, flexc++ defines the enum StartCondition_ defining the names of start-conditions. The %startcondition-name directive can be used to configure another name for the enum containing the names of the start-conditions. It may be defined only once.
The name of the startcondition-enum may be modified, and the directive can also be omitted again after it has been specified before. When changing the name of the startcondition-enum or when reverting to the default name newly generated and ScannerBase.h files will use the currently defined startcondition-enum name. Be advised, though, that the startcondition-enum name may also be used in user-defined members of the scanner-class, or in the scanner’s header and internal header files. If so, the user is responsible for updating those files to the currently defined name of the startcondition-enum.
%target-directory = "pathname"
Pathname defines the directory where generated files should be written. By default this is the directory where flexc++ is called. This directive is overruled by the --target-directory command-line option.
%x namelist
The %x directive is followed by a list of one or more identifiers, separated by blanks. Each identifier is the name of an exclusive start condition.


Mini scanners come in two flavors: inclusive mini scanners and exclusive mini scanners. The rules that apply to an inclusive mini scanner are the mini scanner’s own rules as well as the rules which apply to no mini scanners in particular (i.e., the rules that apply to the default (or INITIAL) mini scanner). Exclusive mini scanners only use the rules that were defined for them.

To define an inclusive mini scanner use %s, followed by one or more identifiers specifying the name(s) of the mini-scanner(s). To define an exclusive mini scanner use %x, followed by or more identifiers specifying the name(s) of the mini-scanner(s). The following example defines the names of two mini scanners: string and comment:

%x string comment
Following this, rules defined in the context of the string mini scanner (see below) will only be used when that mini scanner is active.

A flexc++ input file may contain multiple %s and %x specifications.


Definitions are of the form

identifier  regular-expression

Each definition must be entered on a line of its own. Definitions associate identifiers with regular expressions, allowing the use of ${identifier} as synonym for its regular expression in the rules section of flexc++’s input file. One defined, the identifiers representing regular expressions can also be used in subsequent definitions.


FIRST                   [A-Za-z_]
NAME                    {FIRST}[-A-Za-z0-9_]*


Following directives and definitions a line merely containing two consecutive % characters is expected. Following this line the rules are defined. Rules consist of regular expressions which should be recognized, possibly followed by actions to be executed once a rule’s regular expression has been matched.

If the rule section contains a line starting with two consecutive % characters, then any remaining input is ignored. Note that this second %% separator does not have to be specified. It is purely optional. To specify a regular expression starting with %% surround the %% with double quotes ("%%") or prefix the %% with a blank space: the %%-characters are only considered a separator if they are encountered at the very beginning of a line.


The regular expressions defined in flexc++’s rules files are matched against the information passed to the scanner’s lex function.

Regular expressions begin as the first non-blank character on a line. Comment is interpreted as comment as long as it isn’t part of the regular expresssion. To define a regular expression starting with two slashes (at least) the first slash can be escaped or double quoted. (E.g., "//".* defines C++ comment to end-of-line).

Regular expressions end at the first blank character (to add a blank character, e.g., a space character, to a regular expression, prefix it by a backslash or put it in a double-quoted string).

Actions may be associated with regular expressions. At a match the action that is associated with the regular expression is executed, after which scanning continues when the lexical scanning function (e.g., lex) is called again. Actions are not required, and regular expressions can be defined without any actions at all. If such action-less regular expressions are matched then the match is performed silently, after which processing continues.

Flexc++ tries to match as many characters of the input file as possible (i.e., it uses `greedy matching’). Non-greedy matching is accomplished by a combination of a scanner and parser and/or by using the `lookahead’ operator (/).

The following regular expression `building blocks’ are available. More complex regular expressions are created by combining them:

the character `x’;
any character (byte) except newline;
a character class; in this case, the pattern matches either an `x’, a `y’, or a `z’. See also the paragraph about character classes below;
a character class containing a range; matches an `a’, a `b’, any letter from `j’ through `o’, or a `Z’. See also the paragraph about character classes below;
a negated character class, i.e., any character except for those in the class. In this example, any non-capital character. See also the paragraph about character classes below;
text between double quotes matches the literal string: [xyz]"foo;
the literal string `[xyz]\"foo’ (using a raw string literal);
if X is `a’, `b’, `f’, `n’, `r’, `t’, or `v’, then the ANSI-C interpretation of `\x’ is matched. Otherwise, a literal `X’ is matched (this is used to escape operators such as `*’);
a NUL character (ASCII code 0);
the character with octal value 123;
the character with hexadecimal value 2a;
the regular expression `r’; parentheses are used to override precedence (see below);
the expansion of the `name’ definition;
zero or more regular expressions `r’. This also matches the empty string;
one or more regular expressions `r’;
zero or one regular expression `r’. This also matches the empty string;
the regular expression `r’ followed by the regular expression `s’; called concatenation;
regular expression `r’ at least m, but at most n times (0 <= m <= n). A regular expression to which {0, 0} is appended is ignored, and a warning message is shown.
regular expression `r’ m or more times (0 <= m);
regular expression `r’ exactly m times (0 <= m). A regular expression to which {0} is appended is ignored, and a warning message is shown;
either regular expression `r’ or regular expression `s’;
regular expression `r’ if it is followed by regular expression `s’. The text matched by `s’ is included when determining whether this rule results in the longest match, but `s’ is then returned to the input before the rule’s action (if defined) is executed.
If flexc++ detects patterns potentially not matching any text it generates warnings like this:

[Warning] input, line 7: null-matching regular expression
By placing the comment

on the line just before a regular expression that potentially does not match any text, the warning for that regular expression is suppressed;
a regular expression `r’ at the beginning of a line or file;
a regular expression `r’, occurring at the end of a line. This pattern is identical to `r/\n’;
a regular expression `r’ in start condition `s’;
a regular expression `r’ in start conditions s1, s2, or s3;
a regular expression `r’ in all start conditions;
an end-of-file;
an end-of-file when in start conditions s1 or s2 .

Character classes

Inside a character class all regular expression operators lose their special meanings, except for the escape character (\), the character range operator -, the end of character class operator ], and, at the beginning of the class, ^. All ordinary escape sequences are supported, all other escaped characters are interpreted as literal characters (e.g., \c is a literal c).

To add a closing bracket to a character class use [] or \]. To add a closing bracket to a negated character class use [^] (or use [^ followed by \] somewhere within the character class). Minus characters are used to define character ranges (e.g., [a-d], defining [abcd]) except in the following cases, where flexc++ recognizes a literal minus character: [-, or [^- (a minus at the very beginning of a character class); -] (a minus at the very end of a character class); or \- (an escaped minus character)) Once a character class has started, all subsequent character (ranges) are added to the set, until the final closing bracket (]) has been reached.

Operator precedence

The regular expressions listed above are grouped according to precedence, from highest precedence at the top to lowest at the bottom. From lowest to highest precedence, the operators are:

|: the or-operator at the end of a line (instead of an action) indicates that this expression’s action is identical to the action of the next rule.
/: the look-ahead operator;
|: the or-operator withn a regular expression;
CHAR: individual elements of the regular expression: characters, strings, quoted characters, escaped characters, character sets etc. are all considered CHAR elements. Multiple CHAR elements can be combined by enclosing them in parentheses (e.g., (abc)+ indicates sequences of abc characters, like abcabcabc);
*, ?, +, {: multipliers:
?: zero or one occurrence of the previous element;
+: one or more repetitions of the previous element;
*: zero or more repetitions of the previous element;
{...}: interval specification: a specified number of repetitions of the previous element (see above for specific forms of the interval specification)
{+}, {-}: set operators ({+} computing the union of two sets, {-} computing the difference of the left-hand side set minus the elements in the right-hand side set);

The lex standard defines concatenation as having a higher precedence than the interval expression. This is different from many other regular expression engines, and flexc++ follows these latter engines, giving all `multiplication operators’ equal priority.

Name expansion has the same precedence as grouping (using parentheses to influence the precedence of the other operators in the regular expression). Since the name expansion is treated as a group in flexc++, it is not allowed to use the lookahead operator in a name definition (a named pattern, defined in the definition section).

Predefined sets of characters

Character classes can also contain character class expressions. These are expressions enclosed inside [: and :] delimiters (which themselves must appear between the [ and ] of the character class. Other elements may occur inside the character class as well). The character class expressions are:

[:alnum:] [:alpha:] [:blank:]
[:cntrl:] [:digit:] [:graph:]
[:lower:] [:print:] [:punct:]
[:space:] [:upper:] [:xdigit:]

Character class expressions designate a set of characters equivalent to the corresponding standard C isXXX function. For example, [:alnum:] designates those characters for which isalnum returns true - i.e., any alphabetic or numeric character. For example, the following character classes are all equivalent:


A negated character class such as the example [^A-Z] above will match a newline unless \n (or an equivalent escape sequence) is one of the characters explicitly present in the negated character class (e.g., [^A-Z\n]). This differs from the way many other regular expression tools treat negated character classes, but unfortunately the inconsistency is historically entrenched. Matching newlines means that a pattern like [^"]* can match the entire input unless there’s another quote in the input.

Flexc++ allows negation of character class expressions by prepending ^ to the POSIX character class name.

[:^alnum:] [:^alpha:] [:^blank:]
[:^cntrl:] [:^digit:] [:^graph:]
[:^lower:] [:^print:] [:^punct:]
[:^space:] [:^upper:] [:^xdigit:]

Combining character sets

The {-} operator computes the difference of two character classes. For example, [a-c]{-}[b-z] represents all the characters in the class [a-c] that are not in the class [b-z] (which in this case, is just the single character a). The {-} operator is left associative, so [abc]{-}[b]{-}[c] is the same as [a].

The {+} operator computes the union of two character classes. For example, [a-z]{+}[0-9] is the same as [a-z0-9]. This operator is useful when preceded by the result of a difference operation, as in, [[:alpha:]]{-}[[:lower:]]{+}[q], which is equivalent to [A-Zq] in the C locale.

Trailing context

A rule can have at most one instance of trailing context (the / operator or the $ operator). The start condition, ^, and <<EOF>> patterns can only occur at the beginning of a pattern, and cannot be surrounded by parentheses. The characters ^ and $ only have their special properties at, respectively, the beginning and end of regular expressions. In all other cases they are treated as a normal characters.


%option debug
%x comment
NAME    [[:alpha:]][_[:alnum:]]*
"//".*          // ignore
"/*"            begin(StartCondition_::comment);
<comment>.|\n   // ignore
<comment>"*/"   begin(StartCondition_::INITIAL);
^a              return 1;
a               return 2;
a$              return 3;
{NAME}          return 4;
.|\n            // ignore



Flexc++’s default skeleton files are in /usr/share/flexc++.
By default, flexc++ generates the following files:

Scanner.h: the header file containing the scanner class’s interface.
Scannerbase.h: the header file containing the interface of the scanner class’s base class.
Scanner.ih: the internal header file that is meant to be included by the scanner class’s source files (e.g., it is included by, see the next item’s file), and that should contain all declarations required for compiling the scanner class’s sources. the source file implementing the scanner class member function lex (and support functions), performing the lexical scan.


flexc++(1), flexc++api(3)


The priority of interval expressions ({...}) equals the priority of other multiplicative operators (like *).


This is free software, distributed under the terms of the GNU General Public License (GPL).


Frank B. Brokken (,
Jean-Paul van Oosten (,
Richard Berendsen ( (until 2010).

2008-2020 flexc++.2.08.01.tar.gz