Convert a string containing repeating characters to a string, where each repetition is represented by the character and the number of its copies.
For example, the original string abccccdefffffggghhi converts to the compressed string abc4def5g3h2i.
my $str = 'abccccdefffffggghhi';
$str ~~ s:g/
( (<:alpha>) $0+ )
/{
$0[0] ~ $0.chars
}/;
say $str; # abc4def5g3h2i
The global replacement finds the parts of the string with repeated characters. The tricky part in the regex is the way in which capturing parentheses are counted.
The naïve regex <:alpha>+ matches any letter sequence and consumes the whole string. Thus, only one character must be captured: (<:alpha>). Now, the regex should demand repetitions of that character: $0+, but we also need to capture it as we have to know the length of it.
It is not possible to say (<:alpha>)($0+), as $0 is referring to the capturing part in the second parentheses. The final regex contains nested capturing parentheses. The $0 match object keeps the whole repeated sequence and the array with one element that holds the first matched character. The replacement part uses both elements to build the result: $0[0] ~ $0.chars.