indexOfFirst

Returns index of the first character matching the given predicate, or -1 if the char sequence does not contain such character.

inline fun CharSequence.indexOfFirst(predicate: (Char) -> Boolean): Int(source)

val text = "kotlin123example"
val firstDigitIndex = text.indexOfFirst { it.isDigit() }
println(firstDigitIndex)  // Output: 6

Source