How to split a string and plant the delimiters between the splitted parts in Kotlin?

Question

Say I have a string

"Hello! How do you do? Good day!"

and I want to split it, with my delimiters being: ? and ! using the "split" function the result would be:

`[Hello, How do you do, Good day]`

However, I want it to be:

`[Hello, !, How do you do, ?, Good day, !]`

Answer 1

Here is a similar question in Java: How to split a string, but also keep the delimiters?

Use lookahead. In Kotlin, the code maybe like this:

fun main(args: Array<String>) {
    val str = "Hello! How do you do? Good day!"

    val reg = Regex("(?<=[!?])|(?=[!?])")

    var list = str.split(reg)

    println(list)
}

The output of this is:

[Hello, !, How do you do, ?, Good day, !]

Answer 2

This is my version of such a function:

fun String.splitKeeping(str: String): List<String> {
    return this.split(str).flatMap {listOf(it, str)}.dropLast(1).filterNot {it.isEmpty()}
}

fun String.splitKeeping(vararg strs: String): List<String> {
    var res = listOf(this)
    strs.forEach {str -> 
        res = res.flatMap {it.splitKeeping(str)}
    }
    return res
}

//USAGE:
"Hello! How do you do? Good day!".splitKeeping("!", "?")

It is not very fast (square complexity), but works well for relatively short strings.

Answer 3

Here's an extension function wrapping the code discussed here:

private const val withDelimiter = "((?<=%1\$s)|(?=%1\$s))"

fun Regex.splitWithDelimiter(input: CharSequence) = 
    Regex(withDelimiter.format(this.pattern)).split(input)