Course of Raku / Regexes and grammars / Regexes / Adverbs

Significant whitespace

Inside a regex, spaces are normally ignored — they are only there to make the pattern readable. That is why /foo bar/ matches foobar. Sometimes, though, you really do want a space in the pattern to mean “there is whitespace here”. The :s adverb (short for :sigspace, “significant space”) does exactly that:

say so 'foo    bar' ~~ /:s foo bar/; # True
say so 'foobar'     ~~ /:s foo bar/; # False

With :s, the space between foo and bar in the pattern requires whitespace between the two words in the string. One or more spaces (or tabs) all count, so the run of spaces in 'foo bar' matches.

Significant whitespace becomes especially important with grammars, where it lets you write patterns that read like the language you are parsing. You will meet it again as the difference between a token and a rule.

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