The Ruby Way: Solutions and Techniques in Ruby Programming, Second Edition

3.2. Compiling Regular Expressions

Regular expressions can be compiled using the class method Regexp.compile (which is really only a synonym for Regexp.new). The first parameter is required and may be a string or a regex. (Note that if the parameter is a regex with options, the options will not carry over into the newly compiled regex.)

pat1 = Regexp.compile("^foo.*")  # /^foo.*/
pat2 = Regexp.compile(/bar$/i)   # /bar/ (i not propagated)

The second parameter, if present, is normally a bitwise OR of any of the following constants Regexp::EXTENDED, Regexp::IGNORECASE, and Regexp::MULTILINE. Additionally, any non-nil value will have the result of making the regex case-insensitive; we do not recommend this practice.

options = Regexp::MULTILINE || Regexp::IGNORECASE ...