Scala Pattern Matching Scala String

Question

Scala Pattern Matching Scala String

I want to extract a portion of a string that matches one of the drag regex patterns I have defined:

  //should match R0010, R0100,R0300 etc 
  val rPat="[R]{1}[0-9]{4}".r
  // should match P.25.01.21 , P.27.03.25 etc
  val pPat="[P]{1}[.]{1}[0-9]{2}[.]{1}[0-9]{2}[.]{1}[0-9]{2}".r

When I now define my method to retrieve items as:

  val matcher= (s:String) => s match {case pPat(el)=> println(el) // print the P.25.01.25
                                        case rPat(el)=>println(el) // print R0100 
                                        case _ => println("no match")}

And check it for example:

  val pSt=" P.25.01.21 - Hello whats going on?"
  matcher(pSt)//prints "no match" but should print P.25.01.21
  val rSt= "R0010  test test 3,870" 
  matcher(rSt) //prints also "no match" but should print R0010
  //check if regex is wrong
  val pHead="P.25.01.21"
  pHead.matches(pPat.toString)//returns true
  val rHead="R0010"
  rHead.matches(rPat.toString)//return true

I'm not sure if the regex expression is wrong, but the match method works on elements. So what's wrong with this approach?

+3

scala regex pattern-matching

jojo_Berlin Apr 24 17 at 6:23 am

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3 answers

Inject groups into your templates:

val rPat=".*([R]{1}[0-9]{4}).*".r

val pPat=".*([P]{1}[.]{1}[0-9]{2}[.]{1}[0-9]{2}[.]{1}[0-9]{2}).*".r 

...

scala> matcher(pSt)
P.25.01.21

scala> matcher(rSt)
R0010

+1

Nyavro Apr 24 17 at 6:34 am

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If the code is written like this, the desired output will be generated. The following API documentation: http://www.scala-lang.org/api/2.12.1/scala/util/matching/Regex.html

  //should match R0010, R0100,R0300 etc
  val rPat="[R]{1}[0-9]{4}".r
  // should match P.25.01.21 , P.27.03.25 etc
  val pPat="[P]{1}[.]{1}[0-9]{2}[.]{1}[0-9]{2}[.]{1}[0-9]{2}".r


  def main(args: Array[String]) {
    val pSt=" P.25.01.21 - Hello whats going on?"
    val pPatMatches = pPat.findAllIn(pSt);
    pPatMatches.foreach(println)
    val rSt= "R0010  test test 3,870"
    val rPatMatches = rPat.findAllIn(rSt);
    rPatMatches.foreach(println)

  }

Please let me know if this works for you.

0

Ramakrishna Sharvirala Apr 24 17 at 7:08

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Wiktor Stribiżew · Accepted Answer · 2017-04-24T07:02:46+0000

When you use string pattern matching, you should keep in mind that:

The pattern .r

you pass must match the whole string, otherwise no match will be returned (the solution is to make the pattern .r.unanchored

)
After you make it loose, watch out for unwanted matches: R[0-9]{4}

will match R1234

in CSR123456

(different solutions depend on your actual requirements, usually word boundaries are \b

sufficient or negative lookarounds can be used)
Inside the block, the match

regex match function requires a capture group to be present if you want to return some value (you defined it as el

in pPat(el)

and rPat(el)

.

So, I propose the following solution :

val rPat="""\b(R\d{4})\b""".r.unanchored
val pPat="""\b(P\.\d{2}\.\d{2}\.\d{2})\b""".r.unanchored

val matcher= (s:String) => s match {case pPat(el)=> println(el) // print the P.25.01.25
    case rPat(el)=>println(el) // print R0100 
    case _ => println("no match")
}

Then

val pSt=" P.25.01.21 - Hello whats going on?"
matcher(pSt) // => P.25.01.21
val pSt2_bad=" CP.2334565.01124.212 - Hello whats going on?"
matcher(pSt2_bad) // => no match
val rSt= "R0010  test test 3,870" 
matcher(rSt) // => R0010
val rSt2_bad = "CSR00105  test test 3,870" 
matcher(rSt2_bad) // => no match

Some notes on templates:

\b

- upper word border
(R\d{4})

- capture group corresponding to exactly 4 digits
\b

- end word boundary

Because of the triple quotes used to define a string literal, there is no need to avoid backslashes.

Scala Pattern Matching Scala String

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