c# regular expression match at specific index in string?

Question

I'd like to test if a regex will match part of a string at a specific index (and only starting at that specific index). For example, given the string "one two 3 4 five", I'd like to know that, at index 8, the regular expression [0-9]+ will match "3". RegularExpression.IsMatch and Match both take a starting index, however they both will search the entire rest of the string for a match if necessary.

string text="one two 3 4 five";
Regex num=new Regex("[0-9]+");

//unfortunately num.IsMatch(text,0) also finds a match and returns true
Console.WriteLine("{0} {1}",num.IsMatch(text, 8),num.IsMatch(text,0));

Obviously, I could check if the resulting match starts at the index I am interested in, but I will be doing this a large number of times on large strings, so I don't want to waste time searching for matches later on in the string. Also, I won't know in advance what regular expressions I will actually be testing against the string.

I don't want to:

1. split the string on some boundary like whitespace because in my situation I won't know in advance what a suitable boundary would be
2. have to modify the input string in any way (like getting the substring at index 8 and then using ^ in the regex)
3. search the rest of the string for a match or do anything else that wouldn't be performant for a large number of tests against a large string.

I would like to parse a potentially large user supplied body of text using an arbitrary user supplied grammar. The grammar will be defined in a BNF or PEG like syntax, and the terminals will either be string literals or regular expressions. Thus I will need to check if the next part of the string matches any of the potential terminals as driven by the grammar.

Answer 1

How about using Regex.IsMatch(string, int) using a regular expression starting with \G (meaning "start of last match")?

That appears to work:

using System;
using System.Text.RegularExpressions;

class Test
{
    static void Main()
    {
        string text="one two 3 4 five";
        Regex num=new Regex(@"\G[0-9]+");

        Console.WriteLine("{0} {1}",
                          num.IsMatch(text, 8), // True
                          num.IsMatch(text, 0)); // False
    }
}

Answer 2

If you only want to search a substring of the text, grab that substring before the regex.

myRegex.Match(myString.Substring(8, 10));