Get backreference name-based regex from index-based regex

Question

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?

score 2 · Accepted Answer · answered Aug 08 '18 at 07:17

2

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

(?(<bracket>)

So, try:

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

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

answered Aug 08 '18 at 07:17

CertainPerformance

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I checked again and see that in c# you need to remove `<` and `>` characters in the if condition to make it work. The final regex is `^(?$)?\d+(?(bracket)$)$` – NoName Aug 08 '18 at 07:52
Regex101 does not support .NET regex syntax. It can only show how a regex might work, but cannot be used as a proof the regex will work in the target environment. – Wiktor Stribiżew Aug 08 '18 at 08:02

Wiktor Stribiżew · Answer 2 · 2018-08-08T08:05:08.180

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 )

or

(?( 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

Get backreference name-based regex from index-based regex

2 Answers2