We have 2 conditions: 1. nmatch > AP_MAX_REG_MATCH 2. really big string
For #1 we can either choose to return NULL or "". Since most calls to ap_pregsub() aren't checked, "" might be safer. And, at least, it indicates an error (or can indicate one). For #2 we can choose to also limit the returned string size and only populate the string up to some large but safe limit OR return NULL OR return "" All in all, after really mulling it over, using "" as our default error seems safest for pre-2.4. For 2.4, we should fix our errors to return NULL (and update our own modules to catch that) and fix that API.
