Okay, so much for PCRE match. Can someone show me give me pointers on PCRE replace? Ideal would be something with all the =~ s/// semantics from perl. (Not sure if this is included in Text.Regex.PCRE. is it?) In other words, how to do this with (preferrably) Text.Regex.PCRE ? Of course in this case a POSIX regex is fine, but since PCRE is faster and more expressive, and what I'm used to anyway, PCRE is probably is what I will tend to use by default, unless portability is paramount. **************************** thartman@linodewhyou:~/learning/haskell/UnixTools$ cat lines.txt a b c a b c a b c thartman@linodewhyou:~/learning/haskell/UnixTools$ cat lines.txt | perl -ne '$_ =~ s/(\w) (\w) (\w)/$1 z $3/; print $_' a z c a z c a z c thartman@linodewhyou:~/learning/haskell/UnixTools$