You might just scan your regexes for /\\[a-z]/, but I really don't think in practice you will find anything that actually needs conversion.

Update: Reading more closely, I see a few areas of minor concern for character classes; perl doesn't support collating symbols (e.g. [[.fu.]]) or equivalence class expressions (e.g. [[=à=]]), and perl treats backslash in a character class as an escaping character but egrep should treat it as a literal backslash.