using System;
using System.Text.RegularExpressions;
public class Example
{
public static void Main()
{
string pattern = @"(?<d> [0-9] [-_ –.]* ){4} # subpattern definition
(?=
\g<d>{4} (?: \g<d> (?: \g<d>{3} )? )? # allowed digit sequences
(?: \g<d>+ (*SKIP) (*F) )? # skip the substring if digits remain
)
\g<d>* (?= \g<d>{4} ) # backtrack until there're 4 digits at the end";
string input = @"valid:
12345678
123456789
123456789012
123-456789
123 45 6789
123 – 45 – 6789
invalid:
1234567
1234567890
12345678901
1234567890123
";
RegexOptions options = RegexOptions.CultureInvariant | RegexOptions.IgnorePatternWhitespace;
foreach (Match m in Regex.Matches(input, pattern, options))
{
Console.WriteLine("'{0}' found at index {1}.", m.Value, m.Index);
}
}
}
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for C#, please visit: https://msdn.microsoft.com/en-us/library/system.text.regularexpressions.regex(v=vs.110).aspx