abnf-to-regex generator output (https://github.com/ghostwriter/abnf)

abnf-to-regex.txt

(?(DEFINE)
    # Basic components
    (?<ALPHA>[A-Za-z])
    (?<BIT>[01])
    (?<DIGIT>[0-9])
    (?<HEXDIG>[0-9A-Fa-f])
    (?<CHAR>[\x01-\x7F])
    (?<VCHAR>[\x21-\x7E])
    (?<SP>[\x20])
    (?<WSP>[\x20\x09])
    (?<CR>\x0D)
    (?<LF>\x0A)
    (?<HTAB>\x09)
    (?<DQUOTE>\x22)
    (?<CTL>[\x00-\x1F\x7F])
    (?<OCTET>[\x00-\xFF])
    (?<CRLF>(?:\x0D?\x0A))

    # Whitespace and Comment handling
    (?<C_WSP>(?:(?&WSP)|(?:(?&C_NL)(?&WSP))))
    (?<C_NL>(?:(?&Comment)|(?&CRLF)))
    (?<Comment>[\x3B](?:(?&WSP)|(?&VCHAR))*(?&CRLF))
    (?<LWSP>(?:([\x20\x09]|[\x0D?\x0A][\x20\x09])*))

    # Numerical and Character value definitions
    (?<BinVal>[\x62](?&BIT)+(?:(?:[\x2E](?&BIT)+)+|(?:[\x2D](?&BIT)+))?)
    (?<DecVal>[\x64](?&DIGIT)+(?:(?:[\x2E](?&DIGIT)+)+|(?:[\x2D](?&DIGIT)+))?)
    (?<HexVal>[\x58\x78](?&HEXDIG)+(?:(?:[\x2E](?&HEXDIG)+)+|(?:[\x2D](?&HEXDIG)+))?)
    (?<NumVal>[\x25](?:(?&BinVal)|(?&DecVal)|(?&HexVal)))
    (?<CharVal>[\x22](?:[\x20-\x21]|[\x23-\x7E])*[\x22])
    (?<ProseVal>[\x3C](?:[\x20-\x3D]|[\x3F-\x7E])*[\x3E])

    # Rule identifiers and structure components
    (?<Rulename>(?&ALPHA)(?:(?&ALPHA)|(?&DIGIT)|[\x2D])*)
    (?<DefinedAs>(?:(?&C_WSP)*(?:[\x3D]|[\x3D][\x2F])(?&C_WSP)*))

    # High-level elements
    (?<Element>(?:(?&Rulename)|(?&Group)|(?&Option)|(?&CharVal)|(?&NumVal)|(?&ProseVal)))
    (?<Group>[\x28](?&C_WSP)*(?&Alternation)(?&C_WSP)*[\x29])
    (?<Option>[\x5B](?&C_WSP)*(?&Alternation)(?&C_WSP)*[\x5D])

    # Repetition, Concatenation, and Alternation
    (?<Repeat>(?:(?:(?&DIGIT)+|(?:(?&DIGIT)*[\x2A](?&DIGIT)*))))
    (?<Repetition>(?:(?&Repeat)?(?&Element)))
    (?<Concatenation>(?&Repetition)(?:(?&C_WSP)+(?&Repetition))*)
    (?<Alternation>(?&Concatenation)(?:(?&C_WSP)*[\x2F](?&C_WSP)*(?&Concatenation))*)

    # Full rule and rule list
    (?<Rule>(?&Rulename)(?&DefinedAs)(?&Elements)(?&C_NL))
    (?<Elements>(?&Alternation)(?&C_WSP)*)
    (?<RuleList>(?:(?&Rule)|(?:(?&C_WSP)*(?&C_NL)))+)
)

# Entry point for full rule list
^(?&RuleList)$

with 'ux' modifiers for unicode and enable extended mode

<?php

#BLM

final class ABNFParser
{
    private const ABNF_REGEX = <<<'REGEX'
    (?(DEFINE)
        (?<ALPHA>[A-Za-z])
        (?<DIGIT>[0-9])
        (?<SP>[\x20])
        (?<WSP>[(?&SP)\x09])
        (?<CR>\x0D)
        (?<LF>\x0A)
        (?<Repeat>(?:(?:(?&DIGIT)+(?:(?&DIGIT)*[\x2A](?&DIGIT)*))))
    )
    REGEX;

    public function parse(string $input, string $rule): ?string
    {
        // Build the query pattern dynamically based on the provided rule
        $pattern = sprintf('#%s(?&%s)#Aux', self::ABNF_REGEX, $rule);
        
        if (preg_match($pattern, $input, $matches)) {
            return $matches[0];
        }

        return null;
    }
}

# Usage
$parser = new ABNFParser();

$input = '2*4';
$rule = 'Repeat';
$parsedResult = $parser->parse($input, $rule);

var_dump($parsedResult); // Outputs: string(3) "2*4"

https://3v4l.org/Agmbt Outputs: string(3) "2*4"