regex vs custom replace

import scala.annotation.tailrec
import scala.App

object Test extends App {

  val NumberOfRepetitions = 20

  val longText = "This is a test text containing something and SoMETHING else." * 10000

  println(s"Ready to start. Text size ${longText.length}")

  val t = timeIt(longText.replaceAll("(?i)something", "someother"))

  println(s"Time for regex: $t ms")

  val t2 = timeIt(replaceAllCaseInsensitive(longText, "something", "someother"))

  println(s"Time for custom: $t2 ms")


  def timeIt[T](op: => T) = {
    val t = System.currentTimeMillis
    (1 to NumberOfRepetitions).foreach( _ => op)
    (System.currentTimeMillis - t) / NumberOfRepetitions
  }

  def replaceAllCaseInsensitive(source: String, target: String, replacement: String) = {
    val replacementSize = replacement.size
    val sourceSize = source.size
    val targetSize = target.size
    val sizeDelta = replacementSize - targetSize
    val builder = new StringBuilder(source)
    val end = if(targetSize > 0) sourceSize - targetSize else sourceSize - 1

    @inline
    def findMatchAt(offset: Int): Boolean = {
      var i = 0
      while (i < targetSize) {
        if (source.charAt(i + offset).toLower != target.charAt(i))
          return false
        i += 1
      }
      return true
    }

    @tailrec
    def loop(currentIdx: Int = 0, offset: Int = 0): String = {
      if (currentIdx <= end) {
        if (findMatchAt(currentIdx)) {
          builder.replace(currentIdx + offset, currentIdx + offset + targetSize, replacement)
          loop(currentIdx + targetSize, offset + sizeDelta)
        } else
          loop(currentIdx + 1, offset)
      } else {
        builder.result()
      }
    }

    loop()
  }
}