RegEx - reusing subexpressions

Question

Say I have a regex matching a hexadecimal 32 bit number:

([0-9a-fA-F]{1,8}) 

When I construct a regex where I need to match this multiple times, e.g.

(?<from>[0-9a-fA-F]{1,8})\s*:\s*(?<to>[0-9a-fA-F]{1,8}) 

Do I have to repeat the subexpression definition every time, or is there a way to "name and reuse" it?

I'd imagine something like (warning, invented syntax!)

(?<from>{hexnum=[0-9a-fA-F]{1,8}})\s*:\s*(?<to>{=hexnum}) 

where hexnum= would define the subexpression "hexnum", and {=hexnum} would reuse it.

Since I already learnt it matters: I'm using .NET's System.Text.RegularExpressions.Regex, but a general answer would be interesting, too.

Answer 1

RegEx Subroutines

When you want to use a sub-expression multiple times without rewriting it, you can group it then call it as a subroutine. Subroutines may be called by name, index, or relative position.

Subroutines are supported by PCRE, Perl, Ruby, PHP, Delphi, R, and others. Unfortunately, the .NET Framework is lacking, but there are some PCRE libraries for .NET that you can use instead (such as https://github.com/ltrzesniewski/pcre-net).

Syntax

Here's how subroutines work: let's say you have a sub-expression [abc] that you want to repeat three times in a row.

Standard RegEx
Any: [abc][abc][abc]

Subroutine, by Name
Perl:     (?'name'[abc])(?&name)(?&name)
PCRE: (?P<name>[abc])(?P>name)(?P>name)
Ruby:   (?<name>[abc])\g<name>\g<name>

Subroutine, by Index
Perl/PCRE: ([abc])(?1)(?1)
Ruby:          ([abc])\g<1>\g<1>

Subroutine, by Relative Position
Perl:     ([abc])(?-1)(?-1)
PCRE: ([abc])(?-1)(?-1)
Ruby:   ([abc])\g<-1>\g<-1>

Subroutine, Predefined
This defines a subroutine without executing it.
Perl/PCRE: (?(DEFINE)(?'name'[abc]))(?P>name)(?P>name)(?P>name)

Examples

Matches a valid IPv4 address string, from 0.0.0.0 to 255.255.255.255:
((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))\.(?1)\.(?1)\.(?1)

Without subroutines:
((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))\.((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))\.((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))\.((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))

And to solve the original posted problem:
(?<from>(?P<hexnum>[0-9a-fA-F]{1,8}))\s*:\s*(?<to>(?P>hexnum))

More Info

http://regular-expressions.info/subroutine.html
http://regex101.com/

Answer 2

Why not do something like this, not really shorter but a bit more maintainable.

String.Format("(?<from>{0})\s*:\s*(?<to>{0})", "[0-9a-zA-Z]{1,8}"); 

If you want more self documenting code i would assign the number regex string to a properly named const variable.