What regex can match sequences of the same character?

Question

A friend asked me this and I was stumped: Is there a way to craft a regular expression that matches a sequence of the same character? E.g., match on 'aaa', 'bbb', but not 'abc'?

m|\w{2,3}|  

Wouldn't do the trick as it would match 'abc'.

m|a{2,3}|  

Wouldn't do the trick as it wouldn't match 'bbb', 'ccc', etc.

Answer 1

Sure thing! Grouping and references are your friends:

(.)\1+ 

Will match 2 or more occurences of the same character. For word constituent characters only, use \w instead of ., i.e.:

(\w)\1+ 

Answer 2

Note that in Perl 5.10 we have alternative notations for backreferences as well.

foreach (qw(aaa bbb abc)) {   say;   say ' original' if /(\w)\1+/;   say ' new way'  if /(\w)\g{1}+/;   say ' relative' if /(\w)\g{-1}+/;   say ' named'    if /(?'char'\w)\g{char}+/;   say ' named'    if /(?<char>\w)\k<char>+/; }