Question

In Ruby, if I have two Regexps, I have the possibility to create another regexp like this:

a = /\d+/ # Matches digits
b = /\s+/ # Matches whitespaces
c = Regexp.union(a, b) # Matches sequences that consist only of digits or only of whitespaces

I want to do the same thing in Scala, but I didn’t find out how I could do that. Note that I am not asking for a syntax to create a union of character classes like (\d+)|(\s+) in the previous example, I am really looking for a possibility to create a new Regexp from two given Regexps.

Actually, in the end, I will not do it for just two Regexps but a large number. I don’t care about grouping or anything, I just want to know if a String matches one of a list of given Regexps. I could just check all of them in a loop, but that is too inefficient, that is why I need one Regexp to check the union.

Answer 1

Scala uses the Java regex engine, which is based on the class java.util.regex.Pattern. Pattern has exactly one method that can create a regex:

public static Pattern compile(String regex)

That’s it, and Scala doesn’t give you any relevant enhancements.

But one thing you can do is use the built-in unioning in match statements, here shown with capturing groups in case you want to pull something out of the string:

val Dig = """(\d+)""".r
val Wsp = """(\s+)""".r

scala> "45" match { case Dig(_) | Wsp(_) => println("found"); case _ => }

found

scala> "   " match { case Dig(_) | Wsp(_) => println("found"); case _ => }

found

If you really want a combined regex, you have to do it at the string level. You can get the java Pattern from a Scala regex with .pattern, and another .pattern then gets the string. Most regexes can be wrapped safely in (?:) to get a non-capturing block, so you can combine like so:

val Both = ("(?:"+Dig.pattern.pattern+")|(?:"+Wsp.pattern.pattern+")").r

However, any capturing groups inside will both be represented, but the non-used branch will be null (not exactly a good way to write idiomatic Scala, but anyway, this is what Java uses):

scala> "2" match { case Both(d,w) => if (w!=null) println("white") else println(d) }
2

scala> " " match { case Both(d,w) => if (w!=null) println("white") else println(d) }
white