There seems to be an issue with how the forum displays the less than character (< ) in proximity to certain other characters such as equal to (=). In particular, when attempting to write this regex string:
it was reproduced as
(?< =From: ).*$
Which, of course, didn't work properly when the person I was talking to tried to use it.
I think this is happening because the text entry system converts the keyboard '< ' character into its HTML escape. The escape sequence includes a trailing space which shouldn't actually be rendered. This appears to happen whether or not the characters are inside the HTML pre tags.