Re: Combining 2 preg matches.

Chung Leong wrote:
Rik wrote:
Ah, forgot that in [a-z0-9][_\-][a-z0-9] the character on the right
is already matched, so it won't work as a start for the second _ in
_a_....

You know, I thought that was the problem initially, but then
remembered that the regular expression engine does backtracking in
order to
maximise any match. When it encounters the underscore after assigning
the letter to the first subpattern, it's supposed to abandon the
previous match, backtrack to the letter, and go down the second
branch.

Yes and no. It does exactly what you say, but it is simply not valid:

With the pattern:
'/^(?:[a-z0-9]|[a-z0-9][_\-][a-z0-9])+$/';
one states the entire string can be build by either [a-z0-9](1)OR
[a-z0-9][_\-][a-z0-9](2), think of them as blocks.

Let's examine it (not entirely how it works, but this instance close
enough):
(fixed width font is handy now:)
positions: 123456789012345678901234567890
string: really_a_made_up_string
match1: 111111_error, let's try the other option.
match2: 111112--_error, no other matches possible.

There is no possibility for a match with either (1) or (2) at the second _,
and no other options to match instead at the beginning of the string.

Grtz,
--
Rik Wasmus


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