Merge branch '10864-fix-undefined-with-fallback' into 'master'

Backport: Bring back Gitlab::UntrustedRegexp.with_fallback

See merge request gitlab-org/gitlab-ce!26901

lib/gitlab/untrusted_regexp.rb

@@ -47,6 +47,19 @@ module Gitlab
       self.source == other.source
     end
+    # Handles regular expressions with the preferred RE2 library where possible
+    # via UntustedRegex. Falls back to Ruby's built-in regular expression library
+    # when the syntax would be invalid in RE2.
+    #
+    # One difference between these is `(?m)` multi-line mode. Ruby regex enables
+    # this by default, but also handles `^` and `$` differently.
+    # See: https://www.regular-expressions.info/modifiers.html
+    def self.with_fallback(pattern, multiline: false)
+      UntrustedRegexp.new(pattern, multiline: multiline)
+    rescue RegexpError
+      Regexp.new(pattern)
+    end
+
     private
     attr_reader :regexp