What is the equivalent of branch reset operator (“?|”) found in php(pcre) in C#?

Question

The following regular expression will match "Saturday" or "Sunday" : (?:(Sat)ur|(Sun))day

But in one case backreference 1 is filled while backreference 2 is empty and in the other case vice-versa.

PHP (pcre) provides a nice operator "?|" that circumvents this problem. The previous regex would become (?|(Sat)ur|(Sun))day. So there will not be empty backreferences.

Is there an equivalent in C# or some workaround ?

Answer 1

.NET doesn't support the branch-reset operator, but it does support named groups, and it lets you reuse group names without restriction (something no other flavor does, AFAIK). So you could use this:

(?:(?<abbr>Sat)ur|(?<abbr>Sun))day

...and the abbreviated name will be stored in Match.Groups["abbr"].

Answer 2

should be possible to concat backref1 and backref2.
As one of each is always empty and a string concat with empty is still the same string...

with your regex (?:(Sat)ur|(Sun))day and replacement $1$2
you get Sat for Saturday and Sun for Sunday.

 regex (?:(Sat)ur|(Sun))day
 input    | backref1 _$1_ | backref2 _$2_ | 'concat' _$1$2_
 ---------|---------------|---------------|----------------
 Saturday | 'Sat'         | ''            | 'Sat'+'' = Sat
 Sunday   | ''            | 'Sun'         | ''+'Sun' = Sun

instead of reading backref1 or backref2 just read both results and concat the result.

Answer 3

You can use the branch-reset operator:

(?|foo(bar)|still(life)|(like)so)

That will only set group one no matter which branch matches.