I’ve reorganized the regular expression content in the new Programming Ruby, and added some cool new advanced examples. This one’s fairly straightforward, but I love the fact that I can now start refactoring my more complex patterns, removing duplication.

The stuff below is an extract from the unedited update. It’ll appear in the next beta. It follows a discussion of named groups, \k and related stuff.

There’s a trick which allows us to write subroutines inside regular expressions. Recall that we can invoke a named group using \g<name>, and we define the group using (?<name>...). Normally, the definition of the group is itself matched as part of executing the pattern. However, if you add the suffix {0} to the group, it means “zero matches of this group,” so the group is not executed when first encountered.

sentence = %r{ 
    (?<subject>   cat   | dog   | gerbil    ){0} 
    (?<verb>      eats  | drinks| generates ){0} 
    (?<object>    water | bones | PDFs      ){0} 
    (?<adjective> big   | small | smelly    ){0} 

    (?<opt_adj>   (\g<adjective>\s)?     ){0} 


md = sentence.match("The cat drinks water") 
puts "The subject is #{md[:subject]} and the verb is #{md[:verb]}"
md = sentence.match("The big dog eats smelly bones") 
puts "The adjective in the second sentence is #{md[:adjective]}" 

sentence =~ "The gerbil generates big PDFs" 
puts "And the object in the last is #{$~[:object]}" 


The subject is cat and the verb is drinks 
The adjective in the second sentence is smelly 
And the object in the last is PDFs 

Cool, eh?

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