Is it guaranteed that the compression method field will never nest a space, like "LZW cmp" or something?

yes, I think it's always one word (and probably specifically for easy parsing, judging by the odd names).

When building a regexp against sample data my approach tends to be exactly the opposite of Fletch's

I call the two approaches "Extraction" (/:.{15}(.*)/) and "Validation" (your's). Which I use is determined by the situation. Sometimes, there's a happy middle that's a mixture of both (Fletch's /[[:hexdigit:]]{8}\s+(.*)$/).