There didn't seem to be a predefined source of "combining characters" in the Java standard library. According to the Unicode Character Database 10.0.0 , some of the characters in the "Mn" and "Mc" category fall into the Not_Reordered class, while others fall into one of the real classes. Therefore, I extracted the raw list of code points from that database.

Based on that, I wrote the following code, including some simple test cases. The code works as intended for all test cases I tried. Probably there are some edge cases in other scripts and languages I do not know. I'd like to learn about these, as well as any coding style issues.

package de.roland_illig.strrev
import org.assertj.core.api.Assertions.assertThat
import org.junit.jupiter.api.Test
/**
 * Returns the reversed string, keeping clusters of combining code points
 * (such as German umlauts or Arabic tashkīl) together.
 */
fun String.reverse(): String {
 fun isLamAlef(cluster: List<Int>, ch: Int) =
 cluster.isNotEmpty() && cluster.first() == 0x0644 && ch == 0x0627
 val clusters = mutableListOf<List<Int>>()
 val cluster = mutableListOf<Int>()
 this.codePoints().forEachOrdered { ch ->
 if (!isCombining(ch) && !isLamAlef(cluster, ch)) {
 if (cluster.isNotEmpty()) {
 clusters += cluster.toList()
 cluster.clear()
 }
 }
 cluster += ch
 }
 if (cluster.isNotEmpty()) {
 clusters += cluster.toList()
 cluster.clear()
 }
 return fromCodePoints(*clusters.reversed().flatten().toIntArray())
}
class StringReverseTest {
 @Test
 fun ascii() {
 assertThat("hello".reverse()).isEqualTo("olleh")
 }
 @Test
 fun surrogates() {
 val emoji = fromCodePoints(0x1F645)
 assertThat(emoji.reverse()).isEqualTo(emoji)
 }
 @Test
 fun combining() {
 val combinedUmlaut = fromCodePoints(0x0041, 0x0308)
  assertThat(combinedUmlaut.reverse()).isEqualTo(combinedUmlaut)
 }
 @Test
 fun arabic() {
 assertThat("أَهْلًا وَ سَهْلًا".reverse()).isEqualTo("لًاهْسَ وَ لًاهْأَ")
 }
 @Test
 fun combiningAtBeginning() {
 val combinedUmlaut = fromCodePoints(0x0308, 0x0041)
  assertThat(combinedUmlaut.reverse()).isEqualTo(fromCodePoints(0x0041, 0x0308))
 }
}
/**
 * Generated from Unicode 10.0.0 by:
 *
 * perl -wane \
 * '
 * if (@F > 2 && $F[1] eq ";" && $F[2] != 0) {
 * if ($F[0] =~ /^([0-9a-f]+)\.\.([0-9a-f]+)$/i) {
 * printf("|| codePoint >= 0x%s && codePoint <= 0x%s\n", 1,ドル 2ドル);
 * } else {
 * printf("|| codePoint == 0x%s\n", $F[0]);
 * }
 * }
 * ' DerivedCombiningClass.txt
 */
private fun isCombining(codePoint: Int): Boolean {
 return false
 || codePoint >= 0x0334 && codePoint <= 0x0338
 || codePoint == 0x1CD4
 || codePoint >= 0x1CE2 && codePoint <= 0x1CE8
 || codePoint >= 0x20D2 && codePoint <= 0x20D3
 || codePoint >= 0x20D8 && codePoint <= 0x20DA
 || codePoint >= 0x20E5 && codePoint <= 0x20E6
 || codePoint >= 0x20EA && codePoint <= 0x20EB
 || codePoint == 0x10A39
 || codePoint >= 0x16AF0 && codePoint <= 0x16AF4
 || codePoint == 0x1BC9E
 || codePoint >= 0x1D167 && codePoint <= 0x1D169
 || codePoint == 0x093C
 || codePoint == 0x09BC
 || codePoint == 0x0A3C
 || codePoint == 0x0ABC
 || codePoint == 0x0B3C
 || codePoint == 0x0CBC
 || codePoint == 0x1037
 || codePoint == 0x1B34
 || codePoint == 0x1BE6
 || codePoint == 0x1C37
 || codePoint == 0xA9B3
 || codePoint == 0x110BA
 || codePoint == 0x11173
 || codePoint == 0x111CA
 || codePoint == 0x11236
 || codePoint == 0x112E9
 || codePoint == 0x1133C
 || codePoint == 0x11446
 || codePoint == 0x114C3
 || codePoint == 0x115C0
 || codePoint == 0x116B7
 || codePoint == 0x11D42
 || codePoint == 0x1E94A
 || codePoint >= 0x3099 && codePoint <= 0x309A
 || codePoint == 0x094D
 || codePoint == 0x09CD
 || codePoint == 0x0A4D
 || codePoint == 0x0ACD
 || codePoint == 0x0B4D
 || codePoint == 0x0BCD
 || codePoint == 0x0C4D
 || codePoint == 0x0CCD
 || codePoint >= 0x0D3B && codePoint <= 0x0D3C
 || codePoint == 0x0D4D
 || codePoint == 0x0DCA
 || codePoint == 0x0E3A
 || codePoint == 0x0F84
 || codePoint >= 0x1039 && codePoint <= 0x103A
 || codePoint == 0x1714
 || codePoint == 0x1734
 || codePoint == 0x17D2
 || codePoint == 0x1A60
 || codePoint == 0x1B44
 || codePoint == 0x1BAA
 || codePoint == 0x1BAB
 || codePoint >= 0x1BF2 && codePoint <= 0x1BF3
 || codePoint == 0x2D7F
 || codePoint == 0xA806
 || codePoint == 0xA8C4
 || codePoint == 0xA953
 || codePoint == 0xA9C0
 || codePoint == 0xAAF6
 || codePoint == 0xABED
 || codePoint == 0x10A3F
 || codePoint == 0x11046
 || codePoint == 0x1107F
 || codePoint == 0x110B9
 || codePoint >= 0x11133 && codePoint <= 0x11134
 || codePoint == 0x111C0
 || codePoint == 0x11235
 || codePoint == 0x112EA
 || codePoint == 0x1134D
 || codePoint == 0x11442
 || codePoint == 0x114C2
 || codePoint == 0x115BF
 || codePoint == 0x1163F
 || codePoint == 0x116B6
 || codePoint == 0x1172B
 || codePoint == 0x11A34
 || codePoint == 0x11A47
 || codePoint == 0x11A99
 || codePoint == 0x11C3F
 || codePoint >= 0x11D44 && codePoint <= 0x11D45
 || codePoint == 0x05B0
 || codePoint == 0x05B1
 || codePoint == 0x05B2
 || codePoint == 0x05B3
 || codePoint == 0x05B4
 || codePoint == 0x05B5
 || codePoint == 0x05B6
 || codePoint == 0x05B7
 || codePoint == 0x05B8
 || codePoint == 0x05C7
 || codePoint >= 0x05B9 && codePoint <= 0x05BA
 || codePoint == 0x05BB
 || codePoint == 0x05BC
 || codePoint == 0x05BD
 || codePoint == 0x05BF
 || codePoint == 0x05C1
 || codePoint == 0x05C2
 || codePoint == 0xFB1E
 || codePoint == 0x064B
 || codePoint == 0x08F0
 || codePoint == 0x064C
 || codePoint == 0x08F1
 || codePoint == 0x064D
 || codePoint == 0x08F2
 || codePoint == 0x0618
 || codePoint == 0x064E
 || codePoint == 0x0619
 || codePoint == 0x064F
 || codePoint == 0x061A
 || codePoint == 0x0650
 || codePoint == 0x0651
 || codePoint == 0x0652
 || codePoint == 0x0670
 || codePoint == 0x0711
 || codePoint == 0x0C55
 || codePoint == 0x0C56
 || codePoint >= 0x0E38 && codePoint <= 0x0E39
 || codePoint >= 0x0E48 && codePoint <= 0x0E4B
 || codePoint >= 0x0EB8 && codePoint <= 0x0EB9
 || codePoint >= 0x0EC8 && codePoint <= 0x0ECB
 || codePoint == 0x0F71
 || codePoint == 0x0F72
 || codePoint >= 0x0F7A && codePoint <= 0x0F7D
 || codePoint == 0x0F80
 || codePoint == 0x0F74
 || codePoint >= 0x0321 && codePoint <= 0x0322
 || codePoint >= 0x0327 && codePoint <= 0x0328
 || codePoint == 0x1DD0
 || codePoint == 0x1DCE
 || codePoint == 0x031B
 || codePoint == 0x0F39
 || codePoint >= 0x1D165 && codePoint <= 0x1D166
 || codePoint >= 0x1D16E && codePoint <= 0x1D172
 || codePoint == 0x302A
 || codePoint >= 0x0316 && codePoint <= 0x0319
 || codePoint >= 0x031C && codePoint <= 0x0320
 || codePoint >= 0x0323 && codePoint <= 0x0326
 || codePoint >= 0x0329 && codePoint <= 0x0333
 || codePoint >= 0x0339 && codePoint <= 0x033C
 || codePoint >= 0x0347 && codePoint <= 0x0349
 || codePoint >= 0x034D && codePoint <= 0x034E
 || codePoint >= 0x0353 && codePoint <= 0x0356
 || codePoint >= 0x0359 && codePoint <= 0x035A
 || codePoint == 0x0591
 || codePoint == 0x0596
 || codePoint == 0x059B
 || codePoint >= 0x05A2 && codePoint <= 0x05A7
 || codePoint == 0x05AA
 || codePoint == 0x05C5
 || codePoint >= 0x0655 && codePoint <= 0x0656
 || codePoint == 0x065C
 || codePoint == 0x065F
 || codePoint == 0x06E3
 || codePoint == 0x06EA
 || codePoint == 0x06ED
 || codePoint == 0x0731
 || codePoint == 0x0734
 || codePoint >= 0x0737 && codePoint <= 0x0739
 || codePoint >= 0x073B && codePoint <= 0x073C
 || codePoint == 0x073E
 || codePoint == 0x0742
 || codePoint == 0x0744
 || codePoint == 0x0746
 || codePoint == 0x0748
 || codePoint == 0x07F2
 || codePoint >= 0x0859 && codePoint <= 0x085B
 || codePoint == 0x08E3
 || codePoint == 0x08E6
 || codePoint == 0x08E9
 || codePoint >= 0x08ED && codePoint <= 0x08EF
 || codePoint == 0x08F6
 || codePoint >= 0x08F9 && codePoint <= 0x08FA
 || codePoint == 0x0952
 || codePoint >= 0x0F18 && codePoint <= 0x0F19
 || codePoint == 0x0F35
 || codePoint == 0x0F37
 || codePoint == 0x0FC6
 || codePoint == 0x108D
 || codePoint == 0x193B
 || codePoint == 0x1A18
 || codePoint == 0x1A7F
 || codePoint >= 0x1AB5 && codePoint <= 0x1ABA
 || codePoint == 0x1ABD
 || codePoint == 0x1B6C
 || codePoint >= 0x1CD5 && codePoint <= 0x1CD9
 || codePoint >= 0x1CDC && codePoint <= 0x1CDF
 || codePoint == 0x1CED
 || codePoint == 0x1DC2
 || codePoint == 0x1DCA
 || codePoint == 0x1DCF
 || codePoint == 0x1DF9
 || codePoint == 0x1DFD
 || codePoint == 0x1DFF
 || codePoint == 0x20E8
 || codePoint >= 0x20EC && codePoint <= 0x20EF
 || codePoint >= 0xA92B && codePoint <= 0xA92D
 || codePoint == 0xAAB4
 || codePoint >= 0xFE27 && codePoint <= 0xFE2D
 || codePoint == 0x101FD
 || codePoint == 0x102E0
 || codePoint == 0x10A0D
 || codePoint == 0x10A3A
 || codePoint == 0x10AE6
 || codePoint >= 0x1D17B && codePoint <= 0x1D182
 || codePoint >= 0x1D18A && codePoint <= 0x1D18B
 || codePoint >= 0x1E8D0 && codePoint <= 0x1E8D6
 || codePoint == 0x059A
 || codePoint == 0x05AD
 || codePoint == 0x1939
 || codePoint == 0x302D
 || codePoint >= 0x302E && codePoint <= 0x302F
 || codePoint == 0x1D16D
 || codePoint == 0x05AE
 || codePoint == 0x18A9
 || codePoint >= 0x1DF7 && codePoint <= 0x1DF8
 || codePoint == 0x302B
 || codePoint >= 0x0300 && codePoint <= 0x0314
 || codePoint >= 0x033D && codePoint <= 0x0344
 || codePoint == 0x0346
 || codePoint >= 0x034A && codePoint <= 0x034C
 || codePoint >= 0x0350 && codePoint <= 0x0352
 || codePoint == 0x0357
 || codePoint == 0x035B
 || codePoint >= 0x0363 && codePoint <= 0x036F
 || codePoint >= 0x0483 && codePoint <= 0x0487
 || codePoint >= 0x0592 && codePoint <= 0x0595
 || codePoint >= 0x0597 && codePoint <= 0x0599
 || codePoint >= 0x059C && codePoint <= 0x05A1
 || codePoint >= 0x05A8 && codePoint <= 0x05A9
 || codePoint >= 0x05AB && codePoint <= 0x05AC
 || codePoint == 0x05AF
 || codePoint == 0x05C4
 || codePoint >= 0x0610 && codePoint <= 0x0617
 || codePoint >= 0x0653 && codePoint <= 0x0654
 || codePoint >= 0x0657 && codePoint <= 0x065B
 || codePoint >= 0x065D && codePoint <= 0x065E
 || codePoint >= 0x06D6 && codePoint <= 0x06DC
 || codePoint >= 0x06DF && codePoint <= 0x06E2
 || codePoint == 0x06E4
 || codePoint >= 0x06E7 && codePoint <= 0x06E8
 || codePoint >= 0x06EB && codePoint <= 0x06EC
 || codePoint == 0x0730
 || codePoint >= 0x0732 && codePoint <= 0x0733
 || codePoint >= 0x0735 && codePoint <= 0x0736
 || codePoint == 0x073A
 || codePoint == 0x073D
 || codePoint >= 0x073F && codePoint <= 0x0741
 || codePoint == 0x0743
 || codePoint == 0x0745
 || codePoint == 0x0747
 || codePoint >= 0x0749 && codePoint <= 0x074A
 || codePoint >= 0x07EB && codePoint <= 0x07F1
 || codePoint == 0x07F3
 || codePoint >= 0x0816 && codePoint <= 0x0819
 || codePoint >= 0x081B && codePoint <= 0x0823
 || codePoint >= 0x0825 && codePoint <= 0x0827
 || codePoint >= 0x0829 && codePoint <= 0x082D
 || codePoint >= 0x08D4 && codePoint <= 0x08E1
 || codePoint >= 0x08E4 && codePoint <= 0x08E5
 || codePoint >= 0x08E7 && codePoint <= 0x08E8
 || codePoint >= 0x08EA && codePoint <= 0x08EC
 || codePoint >= 0x08F3 && codePoint <= 0x08F5
 || codePoint >= 0x08F7 && codePoint <= 0x08F8
 || codePoint >= 0x08FB && codePoint <= 0x08FF
 || codePoint == 0x0951
 || codePoint >= 0x0953 && codePoint <= 0x0954
 || codePoint >= 0x0F82 && codePoint <= 0x0F83
 || codePoint >= 0x0F86 && codePoint <= 0x0F87
 || codePoint >= 0x135D && codePoint <= 0x135F
 || codePoint == 0x17DD
 || codePoint == 0x193A
 || codePoint == 0x1A17
 || codePoint >= 0x1A75 && codePoint <= 0x1A7C
 || codePoint >= 0x1AB0 && codePoint <= 0x1AB4
 || codePoint >= 0x1ABB && codePoint <= 0x1ABC
 || codePoint == 0x1B6B
 || codePoint >= 0x1B6D && codePoint <= 0x1B73
 || codePoint >= 0x1CD0 && codePoint <= 0x1CD2
 || codePoint >= 0x1CDA && codePoint <= 0x1CDB
 || codePoint == 0x1CE0
 || codePoint == 0x1CF4
 || codePoint >= 0x1CF8 && codePoint <= 0x1CF9
 || codePoint >= 0x1DC0 && codePoint <= 0x1DC1
 || codePoint >= 0x1DC3 && codePoint <= 0x1DC9
 || codePoint >= 0x1DCB && codePoint <= 0x1DCC
 || codePoint >= 0x1DD1 && codePoint <= 0x1DF5
 || codePoint == 0x1DFB
 || codePoint == 0x1DFE
 || codePoint >= 0x20D0 && codePoint <= 0x20D1
 || codePoint >= 0x20D4 && codePoint <= 0x20D7
 || codePoint >= 0x20DB && codePoint <= 0x20DC
 || codePoint == 0x20E1
 || codePoint == 0x20E7
 || codePoint == 0x20E9
 || codePoint == 0x20F0
 || codePoint >= 0x2CEF && codePoint <= 0x2CF1
 || codePoint >= 0x2DE0 && codePoint <= 0x2DFF
 || codePoint == 0xA66F
 || codePoint >= 0xA674 && codePoint <= 0xA67D
 || codePoint >= 0xA69E && codePoint <= 0xA69F
 || codePoint >= 0xA6F0 && codePoint <= 0xA6F1
 || codePoint >= 0xA8E0 && codePoint <= 0xA8F1
 || codePoint == 0xAAB0
 || codePoint >= 0xAAB2 && codePoint <= 0xAAB3
 || codePoint >= 0xAAB7 && codePoint <= 0xAAB8
 || codePoint >= 0xAABE && codePoint <= 0xAABF
 || codePoint == 0xAAC1
 || codePoint >= 0xFE20 && codePoint <= 0xFE26
 || codePoint >= 0xFE2E && codePoint <= 0xFE2F
 || codePoint >= 0x10376 && codePoint <= 0x1037A
 || codePoint == 0x10A0F
 || codePoint == 0x10A38
 || codePoint == 0x10AE5
 || codePoint >= 0x11100 && codePoint <= 0x11102
 || codePoint >= 0x11366 && codePoint <= 0x1136C
 || codePoint >= 0x11370 && codePoint <= 0x11374
 || codePoint >= 0x16B30 && codePoint <= 0x16B36
 || codePoint >= 0x1D185 && codePoint <= 0x1D189
 || codePoint >= 0x1D1AA && codePoint <= 0x1D1AD
 || codePoint >= 0x1D242 && codePoint <= 0x1D244
 || codePoint >= 0x1E000 && codePoint <= 0x1E006
 || codePoint >= 0x1E008 && codePoint <= 0x1E018
 || codePoint >= 0x1E01B && codePoint <= 0x1E021
 || codePoint >= 0x1E023 && codePoint <= 0x1E024
 || codePoint >= 0x1E026 && codePoint <= 0x1E02A
 || codePoint >= 0x1E944 && codePoint <= 0x1E949
 || codePoint == 0x0315
 || codePoint == 0x031A
 || codePoint == 0x0358
 || codePoint == 0x1DF6
 || codePoint == 0x302C
 || codePoint == 0x035C
 || codePoint == 0x035F
 || codePoint == 0x0362
 || codePoint == 0x1DFC
 || codePoint >= 0x035D && codePoint <= 0x035E
 || codePoint >= 0x0360 && codePoint <= 0x0361
 || codePoint == 0x1DCD
 || codePoint == 0x0345
}

private fun fromCodePoints(vararg codePoints: Int): String =
 String(codePoints, 0, codePoints.size)

The below code works as intended for all test cases I tried. Probably there are some edge cases in other scripts and languages I do not know. I'd like to learn about these, as well as any coding style issues.

package de.roland_illig.strrev
import com.ibm.icu.lang.UCharacter
import org.assertj.core.api.Assertions.assertThat
import org.junit.jupiter.api.Test
/**
 * Returns the reversed string, keeping clusters of combining code points
 * (such as German umlauts or Arabic tashkīl) together.
 */
fun String.reverse(): String {
 fun isLamAlef(cluster: List<Int>, ch: Int) =
 cluster.isNotEmpty() && cluster.first() == 0x0644 && ch == 0x0627
 val clusters = mutableListOf<List<Int>>()
 val cluster = mutableListOf<Int>()
 this.codePoints().forEachOrdered { ch ->
 if (!(UCharacter.getCombiningClass(ch) != 0) && !isLamAlef(cluster, ch)) {
 if (cluster.isNotEmpty()) {
 clusters += cluster.toList()
 cluster.clear()
 }
 }
 cluster += ch
 }
 if (cluster.isNotEmpty()) {
 clusters += cluster.toList()
 cluster.clear()
 }
 return fromCodePoints(*clusters.reversed().flatten().toIntArray())
}
class StringReverseTest {
 @Test
 fun ascii() {
 assertThat("hello".reverse()).isEqualTo("olleh")
 }
 @Test
 fun surrogates() {
 val emoji = fromCodePoints(0x1F645)
 assertThat(emoji.reverse()).isEqualTo(emoji)
 }
 @Test
 fun combining() {
 val combinedUmlaut = fromCodePoints(0x0041, 0x0308)
 assertThat(combinedUmlaut.reverse()).isEqualTo(combinedUmlaut)
 }
 @Test
 fun arabic() {
 assertThat("أَهْلًا وَ سَهْلًا".reverse()).isEqualTo("لًاهْسَ وَ لًاهْأَ")
 }
 @Test
 fun combiningAtBeginning() {
 val combinedUmlaut = fromCodePoints(0x0308, 0x0041)
 assertThat(combinedUmlaut.reverse())
 .isEqualTo(fromCodePoints(0x0041, 0x0308))
 }
}
private fun fromCodePoints(vararg codePoints: Int): String =
 String(codePoints, 0, codePoints.size)

For completeness, here are the Gradle dependencies for build.gradle:

dependencies {
 compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
 compile group: 'com.ibm.icu', name: 'icu4j', version: '61.1'
 testCompile "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
 testCompile "org.junit.jupiter:junit-jupiter-api:5.0.2"
 testCompile "org.assertj:assertj-core:3.9.0"
}