Actually, it's not a constant string. I was trying to say in the OP that I didn't care what the 'xx' was, but I guess I didn't explain that very well.

In reality, most likely, it'll take the form of possible HTML tags. I say possible, because there most likely will be none, but there might be some, and if they're there, they may be either opening or closing tags, or even tags with multiple attributes. I do know that any text will be lowercase. So, for instance, given $foo = 'GOAT', I want all of the following to match:

asdfgGOATaewrjgn
ererGOAskjgbrTslkgjnjng
ccG<!-- comment on status of global geopolitical economics -->OAjgTsvs
aGO<h3>i hate clowns</h3>ATbbb
[download]

And in the last example, the output that I want would be:

'a' . $stuff1 . 'GO<h3>i hate clowns</h3>AT' . $stuff2 . 'bbb'
[download]

I hope that helps.

Matt