RE: [boost] yet another regex partial match bug

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-07-30 00:28:57

John Maddock wrote:
> > BTW, I don't think that explicitly checking for empty 
> > string should do the trick ;) - I believe the current semantics is 
> > flawed and doesn't reflect user's expectations (which are also 
> > supported by the docs).
> 
> Well you won't be surprised to learn that I disagree:-)
> 
> The problem with matching a null string is that any partial match will
> always succeed, since there will always be a null string suffix that
> "matches".  I don't see this as very useful.

For one, it covers our use case :) (input validation). Do you have a use
case in mind there the current semantic (empty string _not_ partially
matching to anything) make sense?

> With respect to validating input, you are right that a null 
> string could match if there was more input, but a null string 
> could match anything at all if there was more input!  

Right, and IMO 'boost::regex_match(..., boost::match_default |
boost::match_partial) should return 'true' in this case.

> To put it another way, why on earth would you want
> to waste CPU cycles trying a partial match against a NULL 
> string?  

For one thing, I hope that when 'boost::regex_match' compares iterators into
input string and discovers that they are equal, it doesn't spent another 5
minutes there just hanging around :). Speaking seriously, it's not at all
clear to me that writing

return input.empty()
    || boost::regex_match(input, match, re, boost::match_default |
boost::match_partial)
    ;

is going to be faster than plain call to properly implemented
'boost::regex_match' (which, after all, can do the same thing inside :)

> The fact that it is NULL tells you that you need some more input 
> before any decision can be taken anyway.

You need explicitly to check for empty string to make that decision. I am
arguing that this is not a user's responsibility and that this should be a
default semantics of partial matches. You don't want users that use
'regex_match' for input validation to always write

input.empty() || boost::regex_match(... boost::match_partial)

instead of simple 

boost::regex_match(... boost::match_partial)

do you?

Aleksey