parse::recdescent.3

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.\" ========================================================================
.\"
.IX Title "Parse::RecDescent 3"
.TH Parse::RecDescent 3 "2003-04-09" "perl v5.10.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
Parse::RecDescent \- Generate Recursive\-Descent Parsers
.SH "VERSION"
.IX Header "VERSION"
This document describes version 1.94 of Parse::RecDescent,
released April  9, 2003.
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& use Parse::RecDescent;
\&
\& # Generate a parser from the specification in $grammar:
\&
\&         $parser = new Parse::RecDescent ($grammar);
\&
\& # Generate a parser from the specification in $othergrammar
\&
\&         $anotherparser = new Parse::RecDescent ($othergrammar);
\&
\&
\& # Parse $text using rule \*(Aqstartrule\*(Aq (which must be
\& # defined in $grammar):
\&
\&        $parser\->startrule($text);
\&
\&
\& # Parse $text using rule \*(Aqotherrule\*(Aq (which must also
\& # be defined in $grammar):
\&
\&         $parser\->otherrule($text);
\&
\&
\& # Change the universal token prefix pattern
\& # (the default is: \*(Aq\es*\*(Aq):
\&
\&        $Parse::RecDescent::skip = \*(Aq[ \et]+\*(Aq;
\&
\&
\& # Replace productions of existing rules (or create new ones)
\& # with the productions defined in $newgrammar:
\&
\&        $parser\->Replace($newgrammar);
\&
\&
\& # Extend existing rules (or create new ones)
\& # by adding extra productions defined in $moregrammar:
\&
\&        $parser\->Extend($moregrammar);
\&
\&
\& # Global flags (useful as command line arguments under \-s):
\&
\&        $::RD_ERRORS       # unless undefined, report fatal errors
\&        $::RD_WARN         # unless undefined, also report non\-fatal problems
\&        $::RD_HINT         # if defined, also suggestion remedies
\&        $::RD_TRACE        # if defined, also trace parsers\*(Aq behaviour
\&        $::RD_AUTOSTUB     # if defined, generates "stubs" for undefined rules
\&        $::RD_AUTOACTION   # if defined, appends specified action to productions
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
.Sh "Overview"
.IX Subsection "Overview"
Parse::RecDescent incrementally generates top-down recursive-descent text
parsers from simple \fIyacc\fR\-like grammar specifications. It provides:
.IP "\(bu" 4
Regular expressions or literal strings as terminals (tokens),
.IP "\(bu" 4
Multiple (non-contiguous) productions for any rule,
.IP "\(bu" 4
Repeated and optional subrules within productions,
.IP "\(bu" 4
Full access to Perl within actions specified as part of the grammar,
.IP "\(bu" 4
Simple automated error reporting during parser generation and parsing,
.IP "\(bu" 4
The ability to commit to, uncommit to, or reject particular
productions during a parse,
.IP "\(bu" 4
The ability to pass data up and down the parse tree (\*(L"down\*(R" via subrule
argument lists, \*(L"up\*(R" via subrule return values)
.IP "\(bu" 4
Incremental extension of the parsing grammar (even during a parse),
.IP "\(bu" 4
Precompilation of parser objects,
.IP "\(bu" 4
User-definable reduce-reduce conflict resolution via
\&\*(L"scoring\*(R" of matching productions.
.ie n .Sh "Using ""Parse::RecDescent"""
.el .Sh "Using \f(CWParse::RecDescent\fP"
.IX Subsection "Using Parse::RecDescent"
Parser objects are created by calling \f(CW\*(C`Parse::RecDescent::new\*(C'\fR, passing in a
grammar specification (see the following subsections). If the grammar is
correct, \f(CW\*(C`new\*(C'\fR returns a blessed reference which can then be used to initiate
parsing through any rule specified in the original grammar. A typical sequence
looks like this:
.PP
.Vb 3
\&        $grammar = q {
\&                        # GRAMMAR SPECIFICATION HERE
\&                     };
\&
\&        $parser = new Parse::RecDescent ($grammar) or die "Bad grammar!\en";
\&
\&        # acquire $text
\&
\&        defined $parser\->startrule($text) or print "Bad text!\en";
.Ve
.PP
The rule through which parsing is initiated must be explicitly defined
in the grammar (i.e. for the above example, the grammar must include a
rule of the form: \*(L"startrule: <subrules>\*(R".
.PP
If the starting rule succeeds, its value (see below)
is returned. Failure to generate the original parser or failure to match a text
is indicated by returning \f(CW\*(C`undef\*(C'\fR. Note that it's easy to set up grammars
that can succeed, but which return a value of 0, \*(L"0\*(R", or "".  So don't be
tempted to write:
.PP
.Vb 1
\&        $parser\->startrule($text) or print "Bad text!\en";
.Ve
.PP
Normally, the parser has no effect on the original text. So in the
previous example the value of \f(CW$text\fR would be unchanged after having
been parsed.
.PP
If, however, the text to be matched is passed by reference:
.PP
.Vb 1
\&        $parser\->startrule(\e$text)
.Ve
.PP
then any text which was consumed during the match will be removed from the
start of \f(CW$text\fR.
.Sh "Rules"
.IX Subsection "Rules"
In the grammar from which the parser is built, rules are specified by
giving an identifier (which must satisfy /[A\-Za\-z]\ew*/), followed by a
colon \fIon the same line\fR, followed by one or more productions,
separated by single vertical bars. The layout of the productions
is entirely free-format:
.PP
.Vb 3
\&        rule1:  production1
\&             |  production2 |
\&                production3 | production4
.Ve
.PP
At any point in the grammar previously defined rules may be extended with
additional productions. This is achieved by redeclaring the rule with the new
productions. Thus:
.PP
.Vb 3
\&        rule1: a | b | c
\&        rule2: d | e | f
\&        rule1: g | h
.Ve
.PP
is exactly equivalent to:
.PP
.Vb 2
\&        rule1: a | b | c | g | h
\&        rule2: d | e | f
.Ve
.PP
Each production in a rule consists of zero or more items, each of which 
may be either: the name of another rule to be matched (a \*(L"subrule\*(R"),
a pattern or string literal to be matched directly (a \*(L"token\*(R"), a
block of Perl code to be executed (an \*(L"action\*(R"), a special instruction
to the parser (a \*(L"directive\*(R"), or a standard Perl comment (which is
ignored).
.PP
A rule matches a text if one of its productions matches. A production
matches if each of its items match consecutive substrings of the
text. The productions of a rule being matched are tried in the same
order that they appear in the original grammar, and the first matching
production terminates the match attempt (successfully). If all
productions are tried and none matches, the match attempt fails.
.PP
Note that this behaviour is quite different from the \*(L"prefer the longer match\*(R"
behaviour of \fIyacc\fR. For example, if \fIyacc\fR were parsing the rule:
.PP
.Vb 2
\&        seq : \*(AqA\*(Aq \*(AqB\*(Aq
\&            | \*(AqA\*(Aq \*(AqB\*(Aq \*(AqC\*(Aq
.Ve
.PP