diff --git a/src/backend/regex/regprefix.c b/src/backend/regex/regprefix.c
index dc96f9ff0a8..a9eb31419dc 100644
--- a/src/backend/regex/regprefix.c
+++ b/src/backend/regex/regprefix.c
@@ -162,14 +162,12 @@ findprefix(struct cnfa * cnfa,
 		thiscolor = COLORLESS;
 		for (ca = cnfa->states[st]; ca->co != COLORLESS; ca++)
 		{
-			/* We ignore lookahead constraints */
-			if (ca->co >= cnfa->ncolors)
-				continue;
-			/* We can also ignore BOS/BOL arcs */
+			/* We can ignore BOS/BOL arcs */
 			if (ca->co == cnfa->bos[0] || ca->co == cnfa->bos[1])
 				continue;
-			/* ... but EOS/EOL arcs terminate the search */
-			if (ca->co == cnfa->eos[0] || ca->co == cnfa->eos[1])
+			/* ... but EOS/EOL arcs terminate the search, as do LACONs */
+			if (ca->co == cnfa->eos[0] || ca->co == cnfa->eos[1] ||
+				ca->co >= cnfa->ncolors)
 			{
 				thiscolor = COLORLESS;
 				break;
diff --git a/src/test/regress/expected/regex.out b/src/test/regress/expected/regex.out
index 320f5e88c37..be151858a38 100644
--- a/src/test/regress/expected/regex.out
+++ b/src/test/regress/expected/regex.out
@@ -153,6 +153,14 @@ explain (costs off) select * from pg_proc where proname ~ '^(abc)?d';
    Filter: (proname ~ '^(abc)?d'::text)
 (2 rows)
 
+explain (costs off) select * from pg_proc where proname ~ '^abcd(x|(?=\w\w)q)';
+                               QUERY PLAN                               
+------------------------------------------------------------------------
+ Index Scan using pg_proc_proname_args_nsp_index on pg_proc
+   Index Cond: ((proname >= 'abcd'::name) AND (proname < 'abce'::name))
+   Filter: (proname ~ '^abcd(x|(?=\w\w)q)'::text)
+(3 rows)
+
 -- Test for infinite loop in pullback() (CVE-2007-4772)
 select 'a' ~ '($|^)*';
  ?column? 
diff --git a/src/test/regress/sql/regex.sql b/src/test/regress/sql/regex.sql
index 5412f6e1c58..c59fa35f24d 100644
--- a/src/test/regress/sql/regex.sql
+++ b/src/test/regress/sql/regex.sql
@@ -34,6 +34,7 @@ explain (costs off) select * from pg_proc where proname ~ '^abc+d';
 explain (costs off) select * from pg_proc where proname ~ '^(abc)(def)';
 explain (costs off) select * from pg_proc where proname ~ '^(abc)$';
 explain (costs off) select * from pg_proc where proname ~ '^(abc)?d';
+explain (costs off) select * from pg_proc where proname ~ '^abcd(x|(?=\w\w)q)';
 
 -- Test for infinite loop in pullback() (CVE-2007-4772)
 select 'a' ~ '($|^)*';