The (?=expr) construct checks whether the text that follows matches expr, without including expr in the result.
This is called positive lookahead.
In the following example, we look for a number followed by the word "miles":
using System;
using System.Text.RegularExpressions;
class Program
{
static void Main(string[] args)
{
Console.WriteLine (Regex.Match ("25 miles", @"\d+\s(?=miles)"));
}
}
The output:
25
The word "miles" was not returned in the result, even though it was required to satisfy the match.
If we append .* to our expression as follows:
using System;
using System.Text.RegularExpressions;
class Program
{
static void Main(string[] args)
{
Console.WriteLine (Regex.Match ("25 miles more", @"\d+\s(?=miles).*"));
}
}
The output:
25 miles more