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Get backreference name-based regex from index-based regex

I have this regex string:

^(()?d+(?(1)))$


It will match following strings:

123
(1234)
1
(33)


But will not match following strings:

(123
123)


I would like to convert the index-based regex to name-based regex, but following regex seem don't work:

^(?<bracket>()?d+(?(k<bracket>)))$


What is equivalent name-based regex for the first regex string?

2 Answers
2

You want an if clause, not a backreference. - k is a backreference. The syntax to test for a previously named subpattern is

k

(?(<bracket>)


So, try:

^(?<bracket>()?d+(?(<bracket>)))$


https://regex101.com/r/Oy9q1H/1

<

>

^(?<bracket>()?d+(?(bracket)))$

To test if a named group stack is not empty, you should use the name itself inside parentherses:

^(?<bracket>()?d+(?(bracket)))$
^^^^^^^^^


See the .NET regex demo

See the Conditional Matching Based on a Valid Captured Group:

This language element attempts to match one of two patterns depending on whether it has matched a specified capturing group. Its syntax is:

(?( name ) yes | no )

(?( name ) yes | no )

or

(?( number ) yes | no )

(?( number ) yes | no )

where name is the name and number is the number of a capturing group, yes is the expression to match if name or number has a match, and no is the optional expression to match if it does not.

See the C# demo:

var strs = new List<string> "123", "(1234)", "1", "(33)", "(123", "123)";
var rx = new Regex(@"^(?<bracket>()?d+(?(bracket)))$");
foreach (var s in strs)
Console.WriteLine($"s => rx.IsMatch(s)");


Output:

123 => True
(1234) => True
1 => True
(33) => True
(123 => False
123) => False


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