RegEx - reusing subexpressions

Question

Say I have a regex matching a hexadecimal 32 bit number:

([0-9a-fA-F]{1,8})

When I construct a regex where I need to match this multiple ti

Answer 1

.NET regex does not support pattern recursion, and if you can use (?(?[0-9a-fA-F]{1,8}))\s*:\s*(?(\g)) in Ruby and PHP/PCRE (where hex is a "technical" named capturing group whose name should not occur in the main pattern), in .NET, you may just define the block(s) as separate variables, and then use them to build a dynamic pattern.

Starting with C#6, you may use an interpolated string literal that looks very much like a PCRE/Onigmo subpattern recursion, but is actually cleaner and has no potential bottleneck when the group is named identically to the "technical" capturing group:

C# demo:

using System;
using System.Text.RegularExpressions;

public class Test
{
    public static void Main()
    {
        var block = "[0-9a-fA-F]{1,8}";
        var pattern = $@"(?{block})\s*:\s*(?{block})";
        Console.WriteLine(Regex.IsMatch("12345678  :87654321", pattern));
    }
}

The $@"..." is a verbatim interpolated string literal, where escape sequences are treated as combinations of a literal backslash and a char after it. Make sure to define literal { with {{ and } with }} (e.g. $@"(?:{block}){{5}}" to repeat a block 5 times).

For older C# versions, use string.Format:

var pattern = string.Format(@"(?{0})\s*:\s*(?{0})", block);

as is suggested in Mattias's answer.