It seems to almost always fail. I created a new project with some actionscript on the first frame, and nothing else:
For the most part, searching for "stop" didn't work, but occasionally - maybe 1 in 20 searches, maybe less - I could get it to find that string of text, when searching for "stop" or "stop();" (without quotes) after toggling the regex checkbox a couple of times. I thought when it worked those couple of times that I had been searching incorrectly, somehow, but using those same settings now no longer finds the text. It is truly bizarre.