From: Brian Candler <b.candler@...>
Date: 2009-04-08T22:23:14+09:00
Subject: Re: Concatenating Regex Smartly

Shak Shak wrote:
> \A\d+\Z  and \A[a-z]+\Z

These are two regular expressions both anchored to the start and end of 
the string.

If you want to match one or the other:

re1 = /\A\d+\z/
re2 = /\A[a-z]+\z/

re3 = /#{re1}|#{re2}/
=> /(?-mix:\A\d+\z)|(?-mix:\A[a-z]+\z)/

But to "concatenate" in the sense of making a regexp which matches 
digits followed by letters, you need to remove the anchors.

re1 = /\d+/
re2 = /[a-z]+/

re3 = /\A#{re1}#{re2}\z/
=> /\A(?-mix:\d+)(?-mix:[a-z]+)\z/

Note that #{re1} and #{re2} are each surrounded by a non-capturing group 
(?...) when they are interpolated into re3. So it should also work 
properly for more complex REs, e.g.

re1 = /a|b/
re2 = /c|d/
re3 = /\A#{re1}#{re2}\z/

But if you want to be extra-certain that it's done correctly, you can 
always add your own additional layer of grouping:

re3 = /\A(?:#{re1}#{re2})\z/
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