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  ERROR     grammar_ko callback returned a string that cannot be converted to a grammar
  ERROR     <luascript/>:9: Errors are trapped...
  ERROR     action_raising_error callback failed
  ERROR     <luascript/>:9: No such function unknown_action
  ERROR     unknown_action callback failed
  NOTICE    ... Grammar parse result: 5

=item Lookahead

Any RHS surrounded by C<(?=...)> or C<(?!...)> is interpreted to a positive or negative lookahead, respectively. Internally, it is processed like a lexeme, and will result in a zero-length lexeme in case of match. You can skip it in the rule valuatio...

  lhs ::= (-(?= /anything/ )-)

will do a positive lookahead of regular expression C</anything/> and skip it in the rule's valuation.

=back

=head2 Grammar meta settings

=over

=item Start rule

By default, the first symbol of a grammar of level C<n> is its start symbol. This can be set once with e.g.:

  :start ::= symbolname

=item Grammar description

By default, a grammar of level C<n> has the description C<Grammar level n>. This can be set once with e.g.:

  :desc ::= 'A single-quoted string'

=item Settings sub grammar

Any setting consist of a reserved keyword, followed by C<< => >> or the Rightwards Double Arrow UTF-8 character C<⇒>, followed by a setting-specific value:

  action => myAction
  action ⇒ myAction

When you use the C<⇒> UTF-8 character, it is recommended to say that the grammar itself is encoded in UTF-8 (c.f. C<marpaESLIFGrammarOption> structure).

=item Defaults

By default, symbol action is C<::transfer> and rule action is C<::concat>, i.e. the parse tree value of a grammar is a binary concatenation of every input representation (see the I<representation> section below), without the eventual discard. Stack m...

  :default ::= action              => defaultRuleAction
               latm                => 1
               discard-is-fallback => 0
               symbol-action       => defaultSymbolAction
               regex-action        => defaultRegexAction
               default-encoding    => UTF-8
               fallback-encoding   => UTF-16

Predefined actions are available for rules and symbols. Please refer the API documentation to know more about value types.

The C<symbol-action> adverb is how a match within a sub-grammar is transfered.

The C<default-encoding> adverb gives default encoding when the recognizer runs in character mode and the end-user gave no encoding.

The C<fallback-encoding> adverb gives fallback encoding when the recognizer runs in character mode and the end-user gave no encoding nor C<default-encoding>. Then MarpaX::ESLIF will try to I<guess> the encoding, and if this fail, will fallback to thi...

The C<discard-is-fallback> adverb indicates ESLIF that C<:discard> should not be tried whenever scan/resume is entered. The default is the safe implementation, i.e. it is always tried, and no grammar terminal will match if C<:discard> matches an equa...

=item Meta actions

=over

=item C<::undef>

Creates a value of type UNDEF.

Meaningful for both rule and symbol actions.

=item C<::ascii>

Creates a value of type STRING, with encoding "ASCII", from the right-hand side representation, guaranteed to be a NUL byte terminated sequence of ASCII characters, or UNDEF if representation is empty. Please refer to the I<representation> section be...

Meaningful for both rule and symbol actions.

=item C<::convert[[^]]+]>

Creates a value of type STRING from the right-hand side representation encoded in the charset specified within the brackets, or UNDEF if representation is empty. I<iconv> convention is used for the charset, i.e. a charset name, followed by eventual o...

Please refer to the I<representation> section below.

Meaningful for both rule and symbol actions.

=item C<::concat>

Creates a value of type ARRAY from the binary concatenatation all the RHS's representation, or UNDEF if representation is empty.

Please refer to the I<representation> section below.

Meaningful for both rule and symbol actions, and is the B<default rule action>.

=item C<::copy[x]>

Copies the RHS number C<x> (first RHS is at indice C<0>), putting UNDEF if it does not exist. This action is the only one that guarantees that the nature of the RHS value is unchanged.

Meaningful only for rule actions.

=item C<::shift>

Alias for C<::copy[0]>.

=item C<::transfer>

Copies the single RHS number value. This action guarantees that the nature of the RHS value is unchanged.

Meaningful only for symbol actions, and is the B<default symbol action>.

=item C<::true>

Creates a value of type BOOL, containing a true value.

Meaningful for both rule and symbol actions.

=item C<::false>

Creates a value of type BOOL, containing a false value.