How do I check if a string is a prefix of any possible other string that matches a given regex.

[Joshua Landau]

On 7 October 2014 17:15,  <jonathan.slenders at gmail.com> wrote:
> Probably I'm turning the use of regular expressions upside down with this question. I don't want to write a regex that matches prefixes of other strings, I know how to do that. I want to generate a regex -- given another regex --, that matches all possible strings that are a prefix of a string that matches the given regex.
> [...]
> Logically, I'd think it should be possible by running the input string against the state machine that the given regex describes, and if at some point all the input characters are consumed, it's a match. (We don't have to run the regex until the end.) But I cannot find any library that does it...

How wide a net are you counting "regular expressions" to be? What
grammar are you using?