Course of Raku / Regexes and grammars / Grammars / What is a grammar

Named regexes

So far you have written patterns directly inside / … /. You can also give a pattern a name and reuse it, just like a subroutine. Declare one with my regex (or my token, which you will meet properly soon):

my regex number { \d+ }

To use a named regex inside another pattern, write its name in angle brackets, <number>:

my regex number { \d+ }

if '42 cats' ~~ / <number> / {
    say $<number>; # ｢42｣
}

Calling <number> both matches the pattern and captures it under that name, so the matched text is available as $<number> — a named capture you get for free.

A named regex can be used several times in one pattern. When it appears more than once, the captures become a list, reached by index:

my regex num { \d+ }

if '3-4' ~~ / <num> '-' <num> / {
    say $<num>[0]; # ｢3｣
    say $<num>[1]; # ｢4｣
}

Named regexes keep patterns readable and let you build larger ones out of smaller, well-named parts. That is exactly what a grammar does on a bigger scale.

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