How to pattern match using regular expression in Scala

Question

I would like to be able to find a match between the first letter of a word  and one of the letters in a group such as  ABC   In pseudocode  this might look something like   case Process word    gt     word firstLetter match         case  a-c  A-C     gt        case     gt           But how do I grab the first letter in Scala instead of Java  How do I express the regular expression properly  Is it possible to do this within a case class

User · Answer

First we should know that regular expression can separately be used. Here is an example:

import scala.util.matching.Regex
val pattern = "Scala".r // <=> val pattern = new Regex("Scala")
val str = "Scala is very cool"
val result = pattern findFirstIn str
result match {
  case Some(v) => println(v)
  case _ =>
} // output: Scala

Second we should notice that combining regular expression with pattern matching would be very powerful. Here is a simple example.

val date = """(\d\d\d\d)-(\d\d)-(\d\d)""".r
"2014-11-20" match {
  case date(year, month, day) => "hello"
} // output: hello

In fact, regular expression itself is already very powerful; the only thing we need to do is to make it more powerful by Scala. Here are more examples in Scala Document: http://www.scala-lang.org/files/archive/api/current/index.html#scala.util.matching.Regex

User · Answer

To expand a little on Andrew s answer  The fact that regular expressions define extractors can be used to decompose the substrings matched by the regex very nicely using Scala s pattern matching  e g    val Process        a-cA-C      s       r    define first  rest is non-space for  p  lt - Process findAllIn  aha bah Cah dah   p match     case Process  b        gt  println  first   a   some rest     case Process    rest    gt  println  some first  rest      rest       etc

User · Answer

String matches is the way to do pattern matching in the regex sense   But as a handy aside  word firstLetter in real Scala code looks like   word 0    Scala treats Strings as a sequence of Char s  so if for some reason you wanted to explicitly get the first character of the String and match it  you could use something like this    Cat  0  toString matches   a-cA-C    res10  Boolean   true   I m not proposing this as the general way to do regex pattern matching  but it s in line with your proposed approach to first find the first character of a String and then match it against a regex   EDIT  To be clear  the way I would do this is  as others have said    Cat  matches    a-cA-C      res14  Boolean   true   Just wanted to show an example as close as possible to your initial pseudocode  Cheers

User · Answer

Since version 2 10  one can use Scala s string interpolation feature   implicit class RegexOps sc  StringContext      def r   new util matching Regex sc parts mkString  sc parts tail map     gt   x           scala gt   123  match   case r  d     gt  true case     gt  false   res34  Boolean   true   Even better one can bind regular expression groups   scala gt   123  match   case r   d   d    gt  d toInt case     gt  0   res36  Int   123  scala gt   10 15  match   case r   d d   first     d d   second     gt  first toInt second toInt case     gt  0   res38  Int   25   It is also possible to set more detailed binding mechanisms   scala gt  object Doubler   def unapply s  String    Some s toInt 2    defined module Doubler  scala gt   10  match   case r   d d   Doubler d      gt  d case     gt  0   res40  Int   20  scala gt  object isPositive   def unapply s  String    s toInt  gt   0   defined module isPositive  scala gt   10  match   case r   d d   d   isPositive       gt  d toInt case     gt  0   res56  Int   10   An impressive example on what s possible with Dynamic is shown in the blog post Introduction to Type Dynamic   object T      class RegexpExtractor params  List String         def unapplySeq str  String          params headOption flatMap    r unapplySeq str         class StartsWithExtractor params  List String         def unapply str  String          params headOption filter  str startsWith    map      gt  str         class MapExtractor keys  List String         def unapplySeq T  map  Map String  T           Some keys map map get            import scala language dynamics    class ExtractorParams params  List String   extends Dynamic       val Map   new MapExtractor params      val StartsWith   new StartsWithExtractor params      val Regexp   new RegexpExtractor params       def selectDynamic name  String          new ExtractorParams params    name         object p extends ExtractorParams Nil     Map  firstName  - gt   John    lastName  - gt   Doe   match       case p firstName lastName Map            Some p Jo StartsWith fn              Some p       w    Regexp lastChar      gt        println s Match   fn     lastChar       case     gt  println  nope

User · Answer

As delnan pointed out  the match keyword in Scala has nothing to do with regexes  To find out whether a string matches a regex  you can use the String matches method  To find out whether a string starts with an a  b or c in lower or upper case  the regex would look like this   word matches   a-cA-C        You can read this regex as  one of the characters a  b  c  A  B or C followed by anything     means  any character  and   means  zero or more times   so      is any string

User · Answer

You can do this because regular expressions define extractors but you need to define the regex pattern first   I don t have access to a Scala REPL to test this but something like this should work   val Pattern      a-cA-C    r word firstLetter match      case Pattern c    gt  c bound to capture group here    case     gt

User · Answer

Note that the approach from  AndrewMyers s answer matches the entire string to the regular expression  with the effect of anchoring the regular expression at both ends of the string using   and    Example   scala gt  val MY RE     foo bar     r MY RE  scala util matching Regex    foo bar     scala gt  val result    foo123  match   case MY RE m    gt  m  case     gt   No match    result  String   foo  scala gt  val result    baz123  match   case MY RE m    gt  m  case     gt   No match    result  String   No match  scala gt  val result    abcfoo123  match   case MY RE m    gt  m  case     gt   No match    result  String   No match   And with no    at the end   scala gt  val MY RE2     foo bar   r MY RE2  scala util matching Regex    foo bar   scala gt  val result    foo123  match   case MY RE2 m    gt  m  case     gt   No match    result  String   No match

[regex] How to pattern match using regular expression in Scala?

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