On 08Aug2017 17:31, Jon Ribbens <jon+use...@unequivocal.eu> wrote:
On 2017-08-08, Chris Angelico <ros...@gmail.com> wrote:
On Wed, Aug 9, 2017 at 2:57 AM, Larry Martell <larry.mart...@gmail.com> wrote:
Yeah, it does not throw for 'A|B|' - but mysql chokes on it with empty
subexpression for regexp' I'd like to flag it before it gets to SQL.


Okay, so your definition of validity is "what MySQL will accept". In
that case, I'd feed it to MySQL and see if it accepts it. Regexps are
sufficiently varied that you really need to use the same engine for
validation as for execution.

... but bear in mind, there have been ways of doing denial-of-service
attacks with valid-but-nasty regexps in the past, and I wouldn't want
to rely on there not being any now.

The ones I've seen still require some input length (I'm thinking exponential rematch backoff stuff here). I suspect that if your test query matches the RE against a fixed empty string it is hard to be exploited. i.e. I think most of this stuff isn't expensive in terms of compiling the regexp but in executing it against text.

Happy to hear to falsifications to my beliefs here.

Cheers,
Cameron Simpson <c...@cskk.id.au> (formerly c...@zip.com.au)
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