The two UI messages masseditregex-page-not-exists and masseditregex-js-pagenotexist read "$1 does not exist." where $1 is replaced with page names.

From usage, I found out that what it really means that the "pattern does not exist in page", or in other words "no match [found]".

The existing could be enhanced to avoid confusion, which is likely to seep into localisations. I can confirm this in Arabic, where localisations are mostly done blindly with little testing, if any.