diff --git a/icu4c/source/i18n/regexcmp.cpp b/icu4c/source/i18n/regexcmp.cpp
index e27c3ca597f..707e212edee 100644
--- a/icu4c/source/i18n/regexcmp.cpp
+++ b/icu4c/source/i18n/regexcmp.cpp
@@ -3463,7 +3463,6 @@ int32_t   RegexCompile::maxMatchLength(int32_t start, int32_t end) {
     U_ASSERT(start <= end);
     U_ASSERT(end < fRXPat->fCompiledPat->size());
 
-
     int32_t    loc;
     int32_t    op;
     int32_t    opType;
@@ -3672,7 +3671,7 @@ int32_t   RegexCompile::maxMatchLength(int32_t start, int32_t end) {
 
         case URX_CTR_LOOP:
         case URX_CTR_LOOP_NG:
-            // These opcodes will be skipped over by code for URX_CRT_INIT.
+            // These opcodes will be skipped over by code for URX_CTR_INIT.
             // We shouldn't encounter them here.
             UPRV_UNREACHABLE;
 
@@ -3700,21 +3699,15 @@ int32_t   RegexCompile::maxMatchLength(int32_t start, int32_t end) {
             {
                 // Look-behind.  Scan forward until the matching look-around end,
                 //   without processing the look-behind block.
-                int32_t  depth = 0;
-                for (;;) {
-                    loc++;
+                int32_t dataLoc = URX_VAL(op);
+                for (loc = loc + 1; loc < end; ++loc) {
                     op = (int32_t)fRXPat->fCompiledPat->elementAti(loc);
-                    if (URX_TYPE(op) == URX_LA_START || URX_TYPE(op) == URX_LB_START) {
-                        depth++;
+                    int32_t opType = URX_TYPE(op);
+                    if ((opType == URX_LA_END || opType == URX_LBN_END) && (URX_VAL(op) == dataLoc)) {
+                        break;
                     }
-                    if (URX_TYPE(op) == URX_LA_END || URX_TYPE(op)==URX_LBN_END) {
-                        if (depth == 0) {
-                            break;
-                        }
-                        depth--;
-                    }
-                    U_ASSERT(loc < end);
                 }
+                U_ASSERT(loc < end);
             }
             break;
 
diff --git a/icu4c/source/test/testdata/regextst.txt b/icu4c/source/test/testdata/regextst.txt
index 677f4822177..759a1963a37 100644
--- a/icu4c/source/test/testdata/regextst.txt
+++ b/icu4c/source/test/testdata/regextst.txt
@@ -1439,6 +1439,8 @@
 "[^\u0000-\U0010ffff]"             "a"
 "[^[^\u0000-\U0010ffff]]"          "<0>a</0>"
 
+"This is a string with (?:one |two |three )endings"   "<0>This is a string with two endings</0>"
+
 # Bug ICU-20544. Similar to 20385, above. Assertion failure with a negative look-behind assertion containing
 #                a set with no contents. Look-behind pattern includes more than just the empty set.
 
@@ -1446,9 +1448,19 @@
 "(?<![^\u0000-\U0010ffff]c)"       "<0></0>abc"
 "(?<=[^[^]]†)"                 "abc"          # Problem also exists w positive look-behind
 
+# Bug ICU-20391. Crash in computation of minimum match length with nested look-around patterns.
+#
+"(?<=(?<=((?=)){0}+)"         E    "aaa"
+"(?<=(?<=((?=)){0}+))"             "<0></0>"
+"(?<=c(?<=b((?=a)){1}+))"          "aaa"
+"abc(?=de(?=f))...g"               "<0>abcdefg</0>"
+"abc(?=de(?=f))...g"               "abcdxfg"
+
+
 #  Random debugging, Temporary
 #
 
+
 #
 #  Regexps from http://www.regexlib.com
 #