How to programmatically remove anchors from a regular expression in Ruby? - ruby

Question

Consider:
regex1 = /\A[a-z0-9\-\_]+\z/
regex2 = remove_anchors(regex1) # => /[a-z0-9\-\_]+/
How to implement a remove_anchors function that programmatically removes any anchors (\A, \z, ^, $) from regex1, producing regex2? Is it even possible to modify an existing regular expression like this in Ruby?

Answer 1

You can use the following function:
def remove_anchors(regex)
pattern = regex.source.gsub(/\A(?:\\A|\^)|(?:\\[zZ]|\$)\z/, '')
return Regexp.new(pattern);
end
And here is an IDEONE demo
The regex literal notation /.../ compiles the regex and its string pattern can be obtained via the source property. With gsub, the anchors like ^, $, \A and \z can be removed from the string pattern.

Answer 2

It is even possible to modify an existing regular expression like this in Ruby?
No, it is not possible to modify an existing Regexp at all in Ruby.
You can just look at the available methods and you will immediately see that there are no mutating methods.
There is exactly one method, which allows you to build a new Regexp from one or more existing Regexps, namely Regexp::union, but that won't help you here.
Pretty much the only thing you can do, is get a String representation of the Regexp using Regexp#to_s, then parse that String, remove the anchors textually, and create a new Regexp from the String via Regexp::new. Note, however, that the syntax of Ruby Regexps is anything but trivial to parse, this is not a simple endeavor.
It appears there is no documentation for the syntax of Ruby's Regexps, so you will have to look at the parser: regparse.c

Answer 3

According to your comments, you're actually trying to use the regular expression from the Semantic gem in your routes:
module Semantic
class Version
SemVerRegexp = /\A(\d+\.\d+\.\d+)(-([0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*))?(\+([0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*))?\Z/
# ...
end
end
According to the routing docs: (you have already tried this)
:constraints takes regular expressions with the restriction that regexp anchors can't be used.
But there's another way: can specify advanced constraints as a lambda. Here's an example:
Rails.application.routes.draw do
get '/some/path/*version_str' => 'versions#show',
format: false,
constraints: lambda { |request|
Semantic::Version::SemVerRegexp =~ request.params[:version_str]
}
end
format: false prevents Rails from extracting trailing dots.
Testing the route in rails console:
r = Rails.application.routes
r.recognize_path '/some/path/1.6.5'
#=> {:controller=>"versions", :action=>"show", :version_str=>"1.6.5"}
r.recognize_path '/some/path/3.7.9-pre.1+revision.15723'
#=> {:controller=>"versions", :action=>"show", :version_str=>"3.7.9-pre.1+revision.15723"}
r.recognize_path '/some/path/123'
#=> ActionController::RoutingError: No route matches "/some/path/123"