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

The substitution operator

The substitution operator is s///. It looks like a match with a second slot: the pattern goes between the first two slashes and the replacement text between the last two:

my $s = 'hello world';
$s ~~ s/world/Raku/;
say $s; # hello Raku

Substitution changes the variable in place, so $s itself is updated. The text in the replacement part is literal — you do not put it in quotes.

Like matching, substitution touches only the first occurrence by default. Add the :g adverb to replace every occurrence:

my $s = 'a-b-c';
$s ~~ s:g/'-'/_/;
say $s; # a_b_c

If you want to keep the original and produce a changed copy instead of modifying in place, use the uppercase S/// operator: it performs the substitution and returns the changed string, leaving the original untouched. Because it does not update a variable in place, point it at one with given rather than ~~:

my $orig = 'color';
say S:g/o/0/ given $orig; # c0l0r
say $orig;                # color

(Writing $orig ~~ S/// warns you to use given instead — with S/// there is nothing to modify in place.)

The same non-destructive behaviour is also available as a method, .subst, which returns a new string:

my $orig = 'color';
say $orig.subst('o', '0', :g); # c0l0r
say $orig;                     # color

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