fun CharSequence.singleOrNull(): Char?
Returns single character, or null if the char sequence is empty or has more than one character.
inline fun CharSequence.singleOrNull( predicate: (Char) -> Boolean ): Char?
Returns the single character matching the given predicate, or null if character was not found or more than one character was found.
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Licensed under the Apache License, Version 2.0.
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.text/single-or-null.html