I tried this with 5.00551 on Solaris 2.6 (egcs 2.91.57) and it failed on the 2nd pattern like the original poster. At first it seemed like the (?-x:...) bit was negating the x flag for the entire regexp, but I tried adding the comment to the pattern and it still failed to match, so I'm not sure. Very odd.

Like another poster, I was thinking that the end of the regexp being on the same line as the comment was causing the /x to be ignored (interpreted as part of the comment), but adding whitespace in there caused the pattern to fail in both cases (odd). Something isn't right.