Utility to functions to convert between camel case and underscore separated names

gistfile1.scala

/**
 * Takes a camel cased identifier name and returns an underscore separated
 * name
 *
 * Example:
 *     camelToUnderscores("thisIsA1Test") == "this_is_a_1_test"
 */
def camelToUnderscores(name: String) = "[A-Z\\d]".r.replaceAllIn(name, {m =>
  "_" + m.group(0).toLowerCase()
})

/* 
 * Takes an underscore separated identifier name and returns a camel cased one
 *
 * Example:
 *    underscoreToCamel("this_is_a_1_test") == "thisIsA1Test"
 */

def underscoreToCamel(name: String) = "_([a-z\\d])".r.replaceAllIn(name, {m =>
  m.group(1).toUpperCase()
})

You shouldn't include digits when converting between camel and underscores, by definition there is no such thing as an upper case digit, and therefore it doesn't make sense to consider a digit a capitalised separate word.

your algorithm doesn't correctly map _FOO_BAR to _FooBar and vice versa; here's my attempt at it:

def underscores2camel(name: String) = {
  assert(!(name endsWith "_"), "names ending in _ not supported by this algorithm")
  "[A-Za-z\\d]+_?|_".r.replaceAllIn(name, { x =>
    val x0 = x.group(0)
    if (x0 == "_") x0
    else x0.stripSuffix("_").toLowerCase.capitalize
  })
}
def camel2underscores(x: String) = {
  "_?[A-Z][a-z\\d]+".r.findAllMatchIn(x).map(_.group(0).toLowerCase).mkString("_")
}

scala> camel2underscores("CamelCaseF")
res37: String = camel_case

@eallik, your algorithm returns "" for a non-camelcased string

scala> camel2underscores("foobar")
res1: String = ""

To avoid the leading _:

  def camelToUnderscores(name: String) = "[A-Z\\d]".r.replaceAllIn(name, {m =>
    if(m.end(0) == 1){
      m.group(0).toLowerCase()
    }else {
      "_" + m.group(0).toLowerCase()
    }
  })

Maybe a little less concise but I prefer this;

def camel2Underscore(text: String) = text.drop(1).foldLeft(text.headOption.map(_.toLower + "") getOrElse "") {
  case (acc, c) if c.isUpper => acc + "_" + c.toLower
  case (acc, c) => acc + c
}

@asheshambasta 👍

I had to support some cases where words were all upper case:

  /**
    * Converts from camelCase to snake_case
    * e.g.: camelCase => camel_case
    *
    * @param name the camelCase name to convert
    * @return snake_case version of the string passed
    */
  def camelToSnake(name: String): String = {
    @tailrec
    def go(accDone: List[Char], acc: List[Char]): List[Char] = acc match {
      case Nil => accDone
      case a::b::c::tail if a.isUpper && b.isUpper && c.isLower => go(accDone ++ List(a, '_', b, c), tail)
      case a::b::tail if a.isLower && b.isUpper => go(accDone ++ List(a, '_', b), tail)
      case a::tail => go(accDone :+ a, tail)
    }
    go(Nil, name.toList).mkString.toLowerCase
  }

  "camelToSnake" should "process right camel case + all upper case + mixed" in {
    camelToSnake("COLUMN") shouldBe "column"
    camelToSnake("someColumnNameRespectingCamel") shouldBe "some_column_name_respecting_camel"
    camelToSnake("columnWITHSomeALLUppercaseWORDS") shouldBe "column_with_some_all_uppercase_words"
  }

@dmateusp 👍 exactly what I needed! Would be great if worked with numeric text:

actual: DirectlyEmployedOr1099Resources => directly_employed_or1099resources
expected: DirectlyEmployedOr1099Resources => directly_employed_or_1099_resources

@dmateusp FYI, that causes a problem with things like camelToSnake("IsAString") => is_astring, when it should be is_a_string.

I prefer not to use regex, this approach runs in n time, it's tail recursive and does not convert consecutive upper chars, e.g. ThisIsCAmel -> this_is_camel:

def camel2Underscore(s: String): String = {
  @tailrec def camel2Underscore(s: String, output: String, lastUppercase: Boolean): String =
    if (s.isEmpty) output
    else {
      val c = if (s.head.isUpper && !lastUppercase) "_" + s.head.toLower else s.head.toLower
      camel2Underscore(s.tail, output + c, s.head.isUpper && !lastUppercase)
    }

  camel2Underscore(s, "", true)
}

@ruloweb This is nice but won't work if the two consecutive uppercase chars are at the start (ie. THisIsCamel) or if there are greater than two consecutive uppercase characters elsewhere (ie. ThisIsCAMel). Consequently, this will fail for converting all caps as well.

I adjusted the code to support the all caps case:

 def camel2Snake(str: String): String = {
    @tailrec
    def camel2SnakeRec(s: String, output: String, lastUppercase: Boolean): String =
      if (s.isEmpty) output
      else {
        val c = if (s.head.isUpper && !lastUppercase) "_" + s.head.toLower else s.head.toLower
        camel2SnakeRec(s.tail, output + c, s.head.isUpper && !lastUppercase)
      }
    if (str.forall(_.isUpper)) str.map(_.toLower)
    else {
      camel2SnakeRec(str, "", true)
    }
  }

@amackillop, it doesnt't work for "THISIsADog" (it gives "t_hi_sis_adog")

i suggest the following code :

def camel2Snake(str: String): String = {

  val headInUpperCase = str.takeWhile(c => c.isUpper || c.isDigit)
  val tailAfterHeadInUppercase = str.dropWhile(c => c.isUpper || c.isDigit)

  if (tailAfterHeadInUppercase.isEmpty) headInUpperCase.toLowerCase else {
    val firstWord = if (!headInUpperCase.dropRight(1).isEmpty) {
      headInUpperCase.last match {
        case c: Any if (c.isDigit) => headInUpperCase
        case _ => headInUpperCase.dropRight(1).toLowerCase
      }
    } else {
      headInUpperCase.toLowerCase + tailAfterHeadInUppercase.takeWhile(c => c.isLower)
    }

    if (firstWord == str.toLowerCase) {
      firstWord
    } else {
      s"${firstWord}_${camel2Snake(str.drop(firstWord.length))}"
    }

  }
}

@0dilon you version stack-overflows if the camelCase already contains an underscore.
e.g. camel2Snake("foo_BarBaz")

Late to the party, but iterate w/ Chars seems fine too if you're ok w/ no handling -s in the Strings

val camelToKebab: String => String = _.foldLeft("") {
      case (acc, chr) if chr.isUpper => acc :+ '-' :+ chr.toLower
      case (acc, chr)                => acc :+ chr
    }

@0dilon you version stack-overflows if the camelCase already contains an underscore.

any update on this @0dilon

val camelToSnake : String => String = _.foldLeft( "" ){ (acc, c) =>
    ( c.isUpper, acc.isEmpty, acc.takeRight(1) == "_" ) match {
      case (true, false, false) => acc + "_" + c.toLower
      case (true, _, _) => acc + c.toLower
      case (false, _, _) => acc + c
    }
  }