Tour of Scala

Regular Expression Patterns

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Regular expressions are strings which can be used to find patterns (or lack thereof) in data. Any string can be converted to a regular expression using the .r method.

import scala.util.matching.Regex

val numberPattern: Regex = "[0-9]".r

numberPattern.findFirstMatchIn("awesomepassword") match {
  case Some(_) => println("Password OK")
  case None => println("Password must contain a number")
}
import scala.util.matching.Regex

val numberPattern: Regex = "[0-9]".r

numberPattern.findFirstMatchIn("awesomepassword") match
  case Some(_) => println("Password OK")
  case None => println("Password must contain a number")

In the above example, the numberPattern is a Regex (regular expression) which we use to make sure a password contains a number.

You can also search for groups of regular expressions using parentheses.

import scala.util.matching.Regex

val keyValPattern: Regex = "([0-9a-zA-Z- ]+): ([0-9a-zA-Z-#()/. ]+)".r

val input: String =
  """background-color: #A03300;
    |background-image: url(img/header100.png);
    |background-position: top center;
    |background-repeat: repeat-x;
    |background-size: 2160px 108px;
    |margin: 0;
    |height: 108px;
    |width: 100%;""".stripMargin

for (patternMatch <- keyValPattern.findAllMatchIn(input))
  println(s"key: ${patternMatch.group(1)} value: ${patternMatch.group(2)}")
import scala.util.matching.Regex

val keyValPattern: Regex = "([0-9a-zA-Z- ]+): ([0-9a-zA-Z-#()/. ]+)".r

val input: String =
  """background-color: #A03300;
    |background-image: url(img/header100.png);
    |background-position: top center;
    |background-repeat: repeat-x;
    |background-size: 2160px 108px;
    |margin: 0;
    |height: 108px;
    |width: 100%;""".stripMargin

for patternMatch <- keyValPattern.findAllMatchIn(input) do
  println(s"key: ${patternMatch.group(1)} value: ${patternMatch.group(2)}")

Here we parse out the keys and values of a String. Each match has a group of sub-matches. Here is the output:

key: background-color value: #A03300
key: background-image value: url(img/header100.png)
key: background-position value: top center
key: background-repeat value: repeat-x
key: background-size value: 2160px 108px
key: margin value: 0
key: height value: 108px
key: width value: 100

Moreover, regular expressions can be used as patterns (in match expressions) to conveniently extract the matched groups:

def saveContactInfomation(contact: String): Unit = {
  import scala.util.matching.Regex

  val emailPattern: Regex = """^(\w+)@(\w+(.\w+)+)$""".r
  val phonePattern: Regex = """^(\d{3}-\d{3}-\d{4})$""".r

  contact match {
    case emailPattern(localPart, domainName, _) => 
      println(s"Hi $localPart, we have saved your email address.")
    case phonePattern(phoneNumber) => 
      println(s"Hi, we have saved your phone number $phoneNumber.")
    case _ => 
      println("Invalid contact information, neither an email address nor phone number.")
  }
}

saveContactInfomation("123-456-7890")
saveContactInfomation("JohnSmith@sample.domain.com")
saveContactInfomation("2 Franklin St, Mars, Milky Way")
def saveContactInfomation(contact: String): Unit =
  import scala.util.matching.Regex

  val emailPattern: Regex = """^(\w+)@(\w+(.\w+)+)$""".r
  val phonePattern: Regex = """^(\d{3}-\d{3}-\d{4})$""".r

  contact match
    case emailPattern(localPart, domainName, _) =>
      println(s"Hi $localPart, we have saved your email address.")
    case phonePattern(phoneNumber) =>
      println(s"Hi, we have saved your phone number $phoneNumber.")
    case _ =>
      println("Invalid contact information, neither an email address nor phone number.")

saveContactInfomation("123-456-7890")
saveContactInfomation("JohnSmith@sample.domain.com")
saveContactInfomation("2 Franklin St, Mars, Milky Way")

The output would be:

Hi, we have saved your phone number 123-456-7890.
Hi JohnSmith, we have saved your email address.
Invalid contact information, neither an email address nor phone number.

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