Course of Raku / Regexes and grammars / Regexes / Adverbs

Adverb scope

The :i adverb can be written in two places, and the difference is where it takes effect. On the operator, m:i/…/ makes the whole pattern case-insensitive. Written inside the regex, :i is positional — it applies only from the point where it appears onward. That lets you relax the case rule for just part of a pattern:

say 'RAKU' ~~ /R :i aku/; # ｢RAKU｣
say 'raku' ~~ /R :i aku/; # Nil

Here the leading R is still matched case-sensitively — so lower-case raku fails — while :i makes only the aku that follows case-insensitive. With m:i/Raku/, by contrast, every letter is case-insensitive, so both RAKU and raku match.

Confined to a group

The effect of an internal adverb is also confined to its enclosing group. In /[:i abc]def/ only abc ignores case; the def after the group is matched strictly:

say 'ABCdef' ~~ /[:i abc]def/; # ｢ABCdef｣
say 'ABCDEF' ~~ /[:i abc]def/; # Nil

Switching an adverb off

To turn an adverb back off partway through a pattern, negate it with a !. So :!i restores case sensitivity from that point on:

say 'ABCdef' ~~ / :i abc :!i def /; # ｢ABCdef｣
say 'ABCDEF' ~~ / :i abc :!i def /; # Nil

:i relaxes the case rule for abc, then :!i puts it back so def must match exactly. Grouping and :!i are two ways to reach the same goal: limit an adverb to just the part of the pattern that needs it. The same on/off switch works for the other in-pattern adverbs too.

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