How to check whether a String fully matches a Regex in Scala?

Question

Assume I have a Regex pattern I want to match many Strings to.

val Digit = """\d""".r

I just want to check whether a given String fully matches the Regex. What is a good and idiomatic way to do this in Scala?

I know that I can pattern match on Regexes, but this is syntactically not very pleasing in this case, because I have no groups to extract:

scala> "5" match { case Digit() => true case _ => false }
res4: Boolean = true

Or I could fall back to the underlying Java pattern:

scala> Digit.pattern.matcher("5").matches
res6: Boolean = true

which is not elegant, either.

Is there a better solution?

• Mygod: I think "5" match { case Digit() => true case _ => false } looks better than using underlying pattern object.

Answer 1

Answering my own question I'll use the "pimp my library pattern"

object RegexUtils {
 implicit class RichRegex(val underlying: Regex) extends AnyVal {
 def matches(s: String) = underlying.pattern.matcher(s).matches
 }
}

and use it like this

import RegexUtils._
val Digit = """\d""".r
if (Digit matches "5") println("match")
else println("no match")

unless someone comes up with a better (standard) solution.

Notes

• I didn't pimp String to limit the scope of potential side effects.

• unapplySeq does not read very well in that context.

• KajMagnus: I pimped with a function misses too. Match and missmatch :-) It's so annoying to have to write !s.matches(r) instead of s misses r. Hmm
• Erik Kaplun: How about the built-in "5" matches "\\d" which @polygenelubricants suggested?
• som-snytt: Data matches a pattern, not vice-versa. The scaladoc on Regex makes a big deal about the lack of a boolean for "matches". Personally, I think you've swapped a nice match for a clunkier if-else. If you don't care about groups, use case r(_*) =>.
• Jameela Huq: There has to be a way to do this without importing an external library...
• som-snytt: @JameelaHuq Folks visiting this question will be pleased with 2.13, where Regex finally gets matches. github.com/scala/scala/pull/6521

Answer 2

I don't know Scala all that well, but it looks like you can just do:

"5".matches("\\d")

References

• http://langref.org/scala/pattern-matching/matching

• mkneissl: Well, that works, but has the disadvantage that the pattern is compiled on every try to match. I'd like to avoid that for performance reasons.
• polygenelubricants: @mkneissl: then it looks like your .pattern.matcher(text).matches is the way to go. You can hide the verbosity under some utility method or overloaded operator or something if Scala supports it.
• mkneissl: Thanks, that's what I am going to do, see my answer. I hope answering one's own questions is accepted behaviour on Stack Overflow... Meta says it is...
• Erik Kaplun: @ed. that's even slower and cruftier, so why?
• Valy Dia: The link given as a reference is broken

Answer 3

For the full match you may use unapplySeq. This method tries to match target (whole match) and returns the matches.

scala> val Digit = """\d""".r
Digit: scala.util.matching.Regex = \d
scala> Digit unapplySeq "1"
res9: Option[List[String]] = Some(List())
scala> Digit unapplySeq "123"
res10: Option[List[String]] = None
scala> Digit unapplySeq "string"
res11: Option[List[String]] = None

• Randall Schulz: While true, the primary use of unapply and unapplySeq is implicitly in the cases of a match block.

Answer 4

 """\d""".r.unapplySeq("5").isDefined //> res1: Boolean = true
 """\d""".r.unapplySeq("a").isDefined //> res2: Boolean = false

• mkneissl: Hmm. Why posting a duplicate of stackoverflow.com/a/3022478/158823 two years later?
• Jack: Your original question asked for a result ending in 'true' or 'false', not 'Some' or 'None'. As far as I'm aware isDefined was not part of the library 2 years ago, but maybe it was. Anyway, my answer is not a duplicate ;-)
• mkneissl: I see, it isn't a duplicate. Sorry.
• Jack: No probs ;-) My mistake, I should have explained why I'm using isDefined in my answer. Just giving code as an answer is generally a bad idea, so it's my bad.

Answer 5

Using Standard Scala library and a pre-compiled regex pattern and pattern matching (which is scala state of the art):

val digit = """(\d)""".r
"2" match {
 case digit( a) => println(a + " is Digit")
 case _ => println("it is something else")
}

more to read: http://www.scala-lang.org/api/2.12.1/scala/util/matching/index.html

Answer 6

The answer is in the regex:

val Digit = """^\d$""".r

Then use the one of the existing methods.

• polygenelubricants: I don't think anchors is the issue here. String/Pattern/Matcher.matches, in Java at least, is whole string match already. I think the issue is just style/idiom for regex-ing in Scala, i.e. what those "one of the existing methods" are.
• Daniel C. Sobral: @polygenelubricants Well, Matcher.matches is an aberration. Ok, it makes some optimizations possible, though I don't know if the Java library actually takes advantage of it. But the standard way for Regular Expressions to express that a full match is required is to use anchors. Since the Scala library does not provide a full match method, then the proper way to do it is to use anchors. Either that, or use the Java library.
• mkneissl: Anchoring is not the problem. See also the "123" example in Vasil's answer.
• mkneissl: @Daniel You might be missing the point -- My question was, if I only need to know if a regex matches fully, what is a good way to express that in Scala. There are a lot of working solutions, but in summary I think there is a method missing in Regex that just does that and nothing else. To answer the question in your commment: The difference from unapplySeq to findFirstMatch is, that I have to change the Regex to add the anchors. Both methods neither immediately express my intent nor return a boolean value, that is I'd have to go from Option to Boolean (no problem, but adding more clutter).
• Daniel C. Sobral: @mkneissl I dislike the concept of Java's matches, but ok. As for Option vs Boolean, add nonEmpty to the end and you'll get the Boolean.