Split string by repeatable delimiter in Java/Kotlin

Question

I am splitting a string by a repeatable delimiter, and am also intended to keep the delimiters as well.

val str = "xxoooooooxxoxoxooooo"
val reg = Regex("(?<=x+)|(?=x+)")
var list = str.split(reg)
println(list)

The output is [, x, x, ooooooo, x, x, o, x, o, x, ooooo], though I would like to get

[xx, ooooooo, xx, o, x, o, x, ooooo]

why don't you just match all [`x+|o+`](https://www.regexplanet.com/share/index.html?share=yyyyd3mtaar) — bobble bubble, Nov 19 '20 at 19:24
@bobblebubble One reason is that if the input string contains not only `x` and `o`, it would not work. However, the string is the question only contains `x` and `o`, so the pattern will work for this string. — Wiktor Stribiżew, Nov 19 '20 at 19:28
@WiktorStribiżew I'm commenting on the sample string and the question, anything else is just your guess. If this is the task, I think using lookarounds is overcomplicated. — bobble bubble, Nov 19 '20 at 19:32

score 5 · Answer 1 · answered Nov 19 '20 at 19:24

5

val str = "xxoooooooxxoxoxooooo"
val reg =  Regex("o+|x+").findAll(str).map { it.value }.toList()
println(reg)
//[xx, ooooooo, xx, o, x, o, x, ooooo]

answered Nov 19 '20 at 19:24

user2424380

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score 4 · Accepted Answer · answered Nov 19 '20 at 19:20

Actually, it is not correct to use + quantifier inside a lookbehind in Java's regex patterns, this is not a documented supported feature. It does not throw exception because internally it is translated into {1,0x7FFFFFFF} and Java's regex supports constrained-width lookbehind patterns. However, this quantifier in both the lookbehind and lookahead makes no difference as these are non-consuming patterns, and the regex engine still checks each position inside a string for a pattern match.

You can use

(?<=x)(?=o)|(?<=o)(?=x)

See a Kotlin demo:

val str = "xxoooooooxxoxoxooooo"
val reg = Regex("(?<=x)(?=o)|(?<=o)(?=x)")
var list = str.split(reg)
println(list) 
// => [xx, ooooooo, xx, o, x, o, x, ooooo]

Split string by repeatable delimiter in Java/Kotlin

2 Answers2