Find all occurrences of character in a Kotlin String

This article explores different ways to find indexes of all occurrences of a character or a substring in a Kotlin String.

1. Find all indexes of a character

The indexOf() function only returns the index of the “first” occurrence of a character within the string, as demonstrated below:

fun main() {

val s = "one, two, three, four, five"

val c = ','

val index = s.indexOf(c)

println(index) // 3

}

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Similarly, the lastIndexOf() function returns the index of the character’s “last” occurrence within a string.

fun main() {

val s = "one, two, three, four, five"

val c = ','

val lastIndex = s.lastIndexOf(c)

println(lastIndex) // 16

}

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To find the index of all instances of a character in a string, efficiently call the indexOf() function repeatedly within a loop. Note that the search for the next index starts from the previous index.

fun main() {

val s = "one, two, three, four, five"

val c = ','

var index = s.indexOf(c)

while (index != -1) {

println(index)

index = s.indexOf(c, index + 1)

}

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Output:

3
8
15
21

This is equivalent to:

fun main() {

val s = "one, two, three, four, five"

val c = ','

var index = -1

while (s.indexOf(c, index + 1).also { index = it } != -1) {

println(index)

}

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Output:

3
8
15
21

2. Find all indexes of a substring

We can use a RegEx to find the indexes of all appearances of a “substring” in a String. A typical implementation of this approach would look like:

fun main() {

val string = "One, Two, Three, Four, Five"

val pattern = "o"

val indices = Regex(pattern).findAll(string)

.map { it.range.first }

.toList()

println(indices) // [7, 18]

}

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To enable case-insensitive matching, specify the IGNORE_CASE regex option.

fun main() {

val string = "One, Two, Three, Four, Five"

val pattern = "o"

val indices = Regex(pattern, RegexOption.IGNORE_CASE).findAll(string)

.map { it.range.first }

.toList()

println(indices) // [0, 7, 18]

}

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We can handle some special inputs like overlapping characters by modifying the Regex to use positive lookahead. For example,

fun main() {

val string = "aaa"

val pattern = "aa"

var indices = Regex(pattern).findAll(string).map { it.range.first }.toList()

println(indices) // [0]

indices = Regex("(?=$pattern)").findAll(string).map { it.range.first }.toList()

println(indices) // [0, 1]

}

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That’s all about finding indexes of all occurrences of a character or a substring in a Kotlin String.

Find all occurrences of character in a Kotlin String

1. Find all indexes of a character

2. Find all indexes of a substring

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