Is it Possible to Perform Addition in a Regex?

Question

Assuming the placeholder $2 is populated with an integer, is it possible to increment it by 1?:

var strReplace = @"$2";
Regex.Replace(strInput, @"((.)*?)", strReplace);

Answer 1

You can use a callback version of Regex.Replace with a MatchEvaluator, see examples at:

• http://msdn.microsoft.com/en-us/library/cft8645c.aspx
• http://www.dotnetperls.com/regex-replace

Here's an example (ideone):

using System;
using System.Text.RegularExpressions;

class Program
{
    static string AddOne(string s)
    {
        return Regex.Replace(s, @"\d+", (match) =>
        {
            long num = 0;
            long.TryParse(match.ToString(), out num);
            return (num + 1).ToString();
        });
    }

    static void Main()
    {
        Console.WriteLine(AddOne("hello 123!"));
        Console.WriteLine(AddOne("bai bai 11"));
    }
}

Output:

hello 124!
bai bai 12

Answer 2

In standard (CS theoretic) regular expressions, it is impossible with a regular expression.

However, Perl and such have extensions to Regular Expressions, which has implications for their behaviour, and I am not familiar enough with them to definitely say that the extended regexes will not do it, but I'm fairly sure that this behaviour is not possible with a regex.