Hi everyone,

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.


E.g. You have the regex  ^[a-z]*4R$  then the strings "a", "ab", "A4" "ab4" are 
prefixes of this regex (because there is a way of adding characters that causes 
the regex to match), but "4a" or "a44" or not.
How do I programmatically create a regex that matches "a", "ab", "A4", etc.. 
but not "4a", "a44", etc..

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...

Thanks a lot, if anyone knows the answer to this question!


Cheers,
Jonathan
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