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I frequently need to convert a String into a Regexp. For many strings, Regexp.new(string) is sufficient. But if string contains special characters, they need to be escaped:

string = "foo(bar)"
regex = Regexp.new(string) # => /foo(bar)/
!!regex.match(string) # => false

The Regexp class has a nice way to escape all characters that are special to regex: Regexp.escape. It's used like so:

string = "foo(bar)"
escaped_string = Regexp.escape(string) # => "foo\\(bar\\)"
regex = Regexp.new(escaped_string) # => /foo\(bar\)/ 
!!regex.match(string) # => true

This really seems like this should be the default way Regexp.new works. Is there a better way to convert a String to a Regexp, besides Regexp.new(Regexp.escape(string))? This is Ruby, after all.

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    Regexp.new should NOT work that way, because one could not use "special" regex constructs then. Also, I think include will do the same job. Check How to check whether a string contains a substring in Ruby?
    – Wiktor Stribiżew
    Commented Apr 20, 2016 at 23:13
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    String::include? is the best way to match a String against another String, but I don't think it can output a Regexp. I buy your point about Regexp.new, though.
    – Simon
    Commented Apr 20, 2016 at 23:24
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    The point is, you do not need a Regexp at all to check if a literal String is present inside another String. That is redundant complication/overhead.
    – Wiktor Stribiżew
    Commented Apr 20, 2016 at 23:27
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    String's [] allows either fixed strings or regexp, as does gsub and sub. An API that is restrictive when the class itself allows either seems too rigid.
    – the Tin Man
    Commented Apr 20, 2016 at 23:52
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    Most regular expression libraries I've used take strings that represent regular expressions and if you have special characters in them then it's your responsibility to escape them either manually, or using the escape method. You could always patch in your own Regexp.string method if you like.
    – tadman
    Commented Apr 21, 2016 at 6:17

1 Answer 1

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You should never need to run Regexp.new(Regexp.escape(string)) This is because, in Core and StdLib, practically every method that takes a Regexp also takes a String (as it should).

In the original case, if you're trying to match a big String big_string on a wacky string with special characters like "foo(bar)", you can just run big_string.match("foo(bar)").

If you're trying to do something fancier, you might need use both ::escape and ::new, but never in direct composition. For example, if we want to match big_string on a wacky string followed by a lone digit, we'll run Regexp.new(Regexp.escape(string) + "\\d").

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  • Be careful here. "\d" is a literal d in string, but /\d/ is a digit in a regular expression context. You need "\\d" in this case.
    – tadman
    Commented Apr 22, 2016 at 14:54
  • Yep, that's what I get for not testing code before posting it.
    – Simon
    Commented Apr 23, 2016 at 5:59
  • I don't think this is correct. "foo(bar)".match("foo(bar)") => nil, but "foo(bar)".match(Regexp.escape("foo(bar)")) => #<MatchData "foo(bar)">
    – Jakob Egger
    Commented Mar 8, 2017 at 15:31

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