Because Ruby uses PCRE, but Perl uses its own regex engine which handles cases like that a lot better. That pattern is rejected right away by Perl's engine beca

Because Ruby uses PCRE, but Perl uses its own regex engine which handles cases like that a lot better. That pattern is rejected right away by Perl's engine because it sees that there isn't another X in that string: anchored "X" at 0 floating "X" at 2..2147483647 (checking floating) minlen 3 Guessing start of match in sv for REx "X(.+)+X" against "=XX===============================" Found floating substr "X" at offset 2... Contradicts anchored substr "X", trying floating at offset 3... Did not find floating substr "X"... Match rejected by optimizer Changing the pattern to /X(.)X/ will cause Perl to do real work. But it'll complete it in 0.003s while Ruby will just hang. I wrote an article a while back about the differences between PCRE and Perl's engine: http://use.perl.org/~avar/journal/33585 http://use.perl.org/~avar/journal/33585