Help understanding Regex name patterns

Question

I'm trying to understand how (?<name>pattern) works in Regex. Is there a good link or can someone offer a simple explanation?

Answer 1

From Mastering Regular Expressions:

Named capture:

\b(?<Area>\d\d\d\)-(?<Exch>\d\d\d)-(?<Num>\d\d\d\d)\b

This "fills the names" Area, Exch, and Num with the components of a US phone number. The program can then refer to each matched substring through its name, for example, RegexObj.Groups("Area") in VB.NET and most other .NET languages, RegexObj.Groups["Area"] in C#, RegexObj.group("Area") in Python, and $matches["Area"] in PHP. The result is clearer code.

Within the regular expression itself, the captured text is available via \k with .NET, and (?P=Area) in Python and PHP.

With Python and .NET (but not with PHP), you can use the same name more than once within the same expression.

Answer 2

This is normally used for back reference problems. See: http://www.regular-expressions.info/named.html

Answer 3

This functionality gives you possibility to easily refer to what you have caught from code (see an example) or from regex itself, using friendly name, instead of an index. A simple example:

Regex regex = new Regex(@"(?<foo>[fF][oO][oO]) \k<foo>");

foreach (Match match in regex.Matches("bar fOO foO foO f0O"))
{
    Console.WriteLine(match);
}

Prints

foO foO

This regex allows you to catch a "foo" with any combination of capital-small letters, but only, if it is preceded by another "foo" with exact the same set of capital-small letters. You can also refer to your group using match.Groups["name"] syntax, so in this example, match.Groups["foo"] will return "foO".

Edit: New example that will use \k<name> syntax.