Perl6 already contains grammars.

Perl5 backported "named captures" from perl6:

  #!/usr/bin/perl
  use warnings;
  use strict;
  use feature qw(switch say);
  #use locale;
  #use re 'debug';

  my $expression = qr{
      (?(DEFINE)
            (?<expr>       (?&space)* (?&term) (?&space)* (?&opterm)? (?&space)* ; (?&space)* )
            (?<term>       (?&identifier)
                           | (?&number)
                           | (?&string)
            )
            (?<opterm>     (?&space)* (?&operator) (?&space)* (?&term) (?&space)* (?&opterm)? (?&space)* )
            (?<operator>   [=+*/-]                                          )
            (?<identifier> [A-Za-z_][A-Za-z0-9_]+            )
            (?<number>     [0-9]+                                           )
            (?<string>     (?&quote) ( (?&char) | (?&bsquote) )*? (?&quote) )
            (?<quote>      \"                                               )
            (?<char>       [^\\\"]                                          )
            (?<bsquote>    \\.                                              )
            (?<space>      [ \t]                                            )
      )
      (?&expr)
    }x;

  while (<>) {
  chomp;
  if ( /^ $expression $/x ) {
    say "Correct expression: $_";
  }else {
    say "Incorrect expression: $_";
  }
  }