Course of Raku / Regexes and grammars / Regexes / Lookaround assertions
Lookbehind
A lookbehind checks what comes immediately before the current position:
<?after …>— succeeds if the text behind matches (positive lookbehind)<!after …>— succeeds if the text behind does not match (negative lookbehind)
This lets you match something only because of what precedes it,
without including that prefix in the result. For example, capture the
digits that follow a dollar sign, but leave the $ out of
the match:
if '$100' ~~ / <?after '$'> \d+ / {
say $/; # ｢100｣
}The <?after '$'> assertion requires a
$ just before the current position, then \d+
matches the digits. The match is 100, without the dollar
sign.
Lookahead and lookbehind are often combined. A piece of text wrapped
in <?after …> and <?before …>
matches only when it sits between the required neighbours — a handy way
to extract a value from a known context.
Practice
Complete the quiz that covers the contents of this topic.
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