I'm using the following regex in c# to match some input cases:
^
(?<entry>[#])?
(?(entry)(?<id>\w+))
(?<value>.*)
$
The options are ignoring pattern whitespaces.
My input looks as follows:
hello
#world
[xxx]
This all can be tested here: DEMO
My problem is that this regex will not match the last line. Why?
What I'm trying to do is to check for an entry character. If it's there I force an identifier by \w+
. The rest of the input should be captured in the last group.
This is a simplyfied regex and simplyfied input.
The problem can be fixed if I change the id regex to something like (?(entry)(?<id>\w+)|)
, (?(entry)(?<id>\w+))?
or (?(entry)(?<id>\w+)?)
.
I try to understand why the conditional group doesn't match as stated in original regex.
I'm firm in regex and know that the regex can be simplyfied to ^(\#(?<id>\w+))?(?<value>.*)$
to match my needs. But the real regex contains two more optional groups:
^
(?<entry>[#])?
(\?\:)?
(\(\?(?:\w+(?:-\w+)?|-\w+)\))?
(?(entry)(?<id>\w+))
(?<value>.*)
$
That's the reason why I'm trying to use a conditional match.
UPDATE 10/12/2018
I tested a little arround it. I found the following regex that should match on every input, even an empty one - but it doesn't:
(?(a)a).*
I'm of the opinion that this is a bug in .net regex and reported it to microsoft: See here for more information