Improve pull request URL regexp

- use a regexp literal and avoid escaping forward slashes
 - escape the period in "github.com"
 - match only hex characters in the commit part
 - allow hyphen in usernames and repo names, matching what we allow for
   tap names
 - avoid unnecessary capture

Library/Contributions/cmd/brew-pull.rb

@@ -29,7 +29,7 @@ ARGV.named.each do |arg|
     end
 
     url = url_match[0]
-    issue = url_match[4]
+    issue = url_match[3]
   end
 
   if tap_name = tap(url)

Library/Homebrew/global.rb

@@ -98,7 +98,7 @@ module Homebrew
   alias_method :failed?, :failed
 end
 
-HOMEBREW_PULL_OR_COMMIT_URL_REGEX = 'https:\/\/github.com\/(\w+)\/homebrew(-\w+)?\/(pull\/(\d+)|commit\/\w{4,40})'
+HOMEBREW_PULL_OR_COMMIT_URL_REGEX = %r[https://github\.com/([\w-]+)/homebrew(-[\w-]+)?/(?:pull/(\d+)|commit/\h{4,40})]
 
 require 'compat' unless ARGV.include? "--no-compat" or ENV['HOMEBREW_NO_COMPAT']