Course of Raku / Regexes and grammars / Regexes / Substitution and replacement

Captures in the replacement

The replacement part of s/// can refer to the captures made by the pattern. This lets you rearrange the matched text rather than just throwing it away.

For example, turn a date written as year-month into month/year by capturing both numbers and putting them back in the other order:

my $d = '2025-06';
$d ~~ s/ (\d+) '-' (\d+) /$1\/$0/;
say $d; # 06/2025

The pattern captures the year into $0 and the month into $1. In the replacement, $1 and $0 are written in the reversed order, with a slash between them. (The slash is escaped as \/ so it is not mistaken for the end of the substitution.) Note that spaces are insignificant on the pattern side but literal on the replacement side, so the replacement is written with no spaces around it.

Named captures work the same way. This makes a substitution read clearly even when there are several pieces:

my $name = 'Doe, Jane';
$name ~~ s/ $<last>=(\w+) ', ' $<first>=(\w+) /$<first> $<last>/;
say $name; # Jane Doe

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