Taciturn

David and I were discussing regexes and he wondered if it was possible to write two regexes so that each one matches the other but not itself. In programmer- and shell-readable terms, given the variables a and b: echo "$a"|(! egrep "$a") && echo "$b"|(! egrep "$b") && echo "$a" |egrep "$b" && echo "$b"|egrep "$a" && echo yeah (Thanks D.). If it says "yeah", you win.

To give it a bit more of a concrete scope, I decided that it should use extended regexes, because they have a good balance of power, usefulness, and sanity. Secondly, to avoid the easy solution of ^a and a$, both regexes must be anchored at both beginning and end.

To give readers a chance to solve this themselves, I'll disguise the spoilers below in white-on-white text. Highlight the text to read it.

Doing the same when anchors are prohibited seems to be more difficult, although I haven't thought about it long enough to be satisfied that it's impossible.

By the way, setting GREP_OPTIONS='--color=auto' and optionally also --exclude-dir=.svn is totally awesome.