Question

We allow some user-supplied REs for the purpose of filtering email. Early on we ran into some performance issues with REs that contained, for example, .*, when matching against arbitrarily-large emails. We found a simple solution was to s/\*/{0,1024}/ on the user-supplied RE. However, this is not a perfect solution, as it will break with the following pattern:

/[*]/

And rather than coming up with some convoluted recipe to account for every possible mutation of user-supplied RE input, I’d like to just limit perl’s interpretation of the * and + characters to have a maximum length of 1024 characters.

Is there any way to do this?

Answer 1

Update

Added a (?<!\\) before the quantifiers, because escaped *+ should not be matched. Replacement will still fail if there is an \\* (match \ 0 or more times).

An improvement would be this

s/(?<!\\)\*(?!(?<!\\)[^[]*?(?<!\\)\])/{0,1024}/
s/(?<!\\)\+(?!(?<!\\)[^[]*?(?<!\\)\])/{1,1024}/

See it here on Regexr

That means match [*+] but only if there is no closing ] ahead and no [ till then. And there is no \ (the (?<!\\) part) allowed before the square brackets.

(?! ... ) is a negative lookahead

(?<! ... ) is a negative lookbehind

See perlretut for details

Update 2 include possessive quantifiers

s/(?<!(?<!\\)[\\+*?])\+(?!(?<!\\)[^[]*?(?<!\\)\])/{1,1024}/   # for +
s/(?<!\\)\*(?!(?<!\\)[^[]*?(?<!\\)\])/{0,1024}/    # for *

See it here on Regexr

Seems to be working, but its getting real complicated now!