Question

I would like to compose regexps, that is reusing a regex in a new regex.

Is that possible in Ruby ?

For instance, in order to simplify this assembly-like parsing :

LABELED_INSTR = /(\w+):(movi|addi)\s+(\w+),(\w+),(w+)/
NON_LABELED_INSTR = /(movi|addi)\s+(\w+),(\w+),(w+)/

I would like to resort to :

IMMEDIATE = /(movi|addi)/

But then I don’t know how to share this regex in the two previous ones.

Any hint ?

Answer 1

Sure, regular expressions can be reused (or composed) within other regexes. Here’s an example that combines two regexes to make a third:

>> a = /boo/
=> boo
>> b = /foo/
=> foo
>> c = /#{a}|#{b}/
=> -mix:boo-mix:foo
>> if "boo" =~ c
>>   puts "match!"
>>   end
match!
=> nil

Your example is pretty similar. Here, it would be:

IMMEDIATE = /(movi|addi)/
LABELED_INSTR = /(\w+):#{IMMEDIATE}\s+(\w+),(\w+),(w+)/
NON_LABELED_INSTR = /#{IMMEDIATE}\s+(\w+),(\w+),(w+)/