Do not validate a backreference if the group that it refers to is not valid

regex/matching.go

@@ -319,7 +319,7 @@ func findAllSubmatchHelper(start *nfaState, str []rune, offset int, numGroups in
 				if currentState.contentContains(str, idx, preferLongest) {
 					nextStates = addStateToList(str, idx+1, nextStates, *currentState.next, currentState.threadGroups, nil, preferLongest)
 				}
-			} else if currentState.isBackreference {
+			} else if currentState.isBackreference && currentState.threadGroups[currentState.referredGroup].IsValid() {
 				groupLength := currentState.threadGroups[currentState.referredGroup].EndIdx - currentState.threadGroups[currentState.referredGroup].StartIdx
 				if currentState.threadBackref == groupLength {
 					currentState.threadBackref = 0

regex/re_test.go

@@ -314,10 +314,6 @@ var reTests = []struct {
 	{`\0009`, nil, "\x009", []Group{{0, 2}}},
 	{`\0141`, nil, "a", []Group{{0, 1}}},
 
-	// At this point, the python test suite has a bunch
-	// of backreference tests. Since my engine doesn't
-	// implement backreferences, I've skipped those tests.
-
 	{`*a`, nil, ``, nil},
 	{`(*)b`, nil, ``, nil},
 	{`a**`, nil, ``, nil},
@@ -585,9 +581,29 @@ var groupTests = []struct {
 	{`(.*)c(.*)`, nil, `abcde`, []Match{[]Group{{0, 5}, {0, 2}, {3, 5}}}},
 	{`\((.*), (.*)\)`, nil, `(a, b)`, []Match{[]Group{{0, 6}, {1, 2}, {4, 5}}}},
 
-	// At this point, the python test suite has a bunch
-	// of backreference tests. Since my engine doesn't
-	// implement backreferences, I've skipped those tests.
+	// Backreference tests
+	{`(abc)\1`, nil, `abcabc`, []Match{[]Group{{0, 6}, {0, 3}}}},
+	{`([a-c]+)\1`, nil, `abcabc`, []Match{[]Group{{0, 6}, {0, 3}}}},
+	{`([a-c]*)\1`, nil, `abcabc`, []Match{[]Group{{0, 6}, {0, 3}}, []Group{{6, 6}, {6, 6}}}},
+	{`^(.+)?B`, nil, `AB`, []Match{[]Group{{0, 2}, {0, 1}}}},
+	{`(a+).\1$`, nil, `aaaaa`, []Match{[]Group{{0, 5}, {0, 2}}}},
+	{`^(a+).\1$`, nil, `aaaa`, []Match{}},
+	{`(a)\1`, nil, `aa`, []Match{[]Group{{0, 2}, {0, 1}}}},
+	{`(a+)\1`, nil, `aa`, []Match{[]Group{{0, 2}, {0, 1}}}},
+	{`(a+)+\1`, nil, `aa`, []Match{[]Group{{0, 2}, {0, 1}}}},
+	{`(a).+\1`, nil, `aba`, []Match{[]Group{{0, 3}, {0, 1}}}},
+	{`(a)ba*\1`, nil, `aba`, []Match{[]Group{{0, 3}, {0, 1}}}},
+	{`(aa|a)a\1$`, nil, `aaa`, []Match{[]Group{{0, 3}, {0, 1}}}},
+	{`(a|aa)a\1$`, nil, `aaa`, []Match{[]Group{{0, 3}, {0, 1}}}},
+	{`(a+)a\1$`, nil, `aaa`, []Match{[]Group{{0, 3}, {0, 1}}}},
+	{`([abc]*)\1`, nil, `abcabc`, []Match{[]Group{{0, 6}, {0, 3}}, []Group{{6, 6}, {6, 6}}}},
+	{`(a)(?:b)\1`, nil, `aba`, []Match{[]Group{{0, 3}, {0, 1}}}},
+	{`(a)(?:b)\1`, nil, `abb`, []Match{}},
+	{`(?:a)(b)\1`, nil, `aba`, []Match{}},
+	{`(?:a)(b)\1`, nil, `abb`, []Match{[]Group{{0, 3}, {1, 2}}}},
+	{`(?:(cat)|(dog))\2`, nil, `catdog`, []Match{}},
+	{`(?:a)\1`, nil, `aa`, nil},
+	{`((cat)|(dog)|(cow)|(bat))\4`, nil, `cowcow`, []Match{[]Group{{0, 6}, {0, 3}, {-1, -1}, {-1, -1}, {0, 3}, {-1, -1}}}},
 
 	{`(a)(b)c|ab`, nil, `ab`, []Match{[]Group{{0, 2}}}},
 	{`(a)+x`, nil, `aaax`, []Match{[]Group{{0, 4}, {2, 3}}}},
@@ -792,7 +808,7 @@ func TestFindSubmatch(t *testing.T) {
 				if test.result != nil {
 					panic(err)
 				}
-			}
+			} else {
 				match, err := regComp.FindSubmatch(test.str)
 				if err != nil {
 					if len(test.result) != 0 {
@@ -812,6 +828,7 @@ func TestFindSubmatch(t *testing.T) {
 						}
 					}
 				}
+			}
 		})
 	}
 }
@@ -823,7 +840,7 @@ func TestFindStringSubmatch(t *testing.T) {
 				if test.result != nil {
 					panic(err)
 				}
-			}
+			} else {
 				matchStr := regComp.FindStringSubmatch(test.str)
 				if matchStr == nil {
 					if len(test.result) != 0 {
@@ -858,6 +875,7 @@ func TestFindStringSubmatch(t *testing.T) {
 						}
 					}
 				}
+			}
 		})
 	}
 }
@@ -870,7 +888,7 @@ func TestFindAllStringSubmatch(t *testing.T) {
 				if test.result != nil {
 					panic(err)
 				}
-			}
+			} else {
 				matchStrs := regComp.FindAllStringSubmatch(test.str)
 				if matchStrs == nil {
 					if len(test.result) != 0 {
@@ -911,6 +929,7 @@ func TestFindAllStringSubmatch(t *testing.T) {
 						}
 					}
 				}
+			}
 		})
 	}
 }
@@ -923,7 +942,7 @@ func TestFindAllSubmatch(t *testing.T) {
 				if test.result != nil {
 					panic(err)
 				}
-			}
+			} else {
 				matchIndices := regComp.FindAllSubmatch(test.str)
 				for i := range matchIndices {
 					for j := range matchIndices[i] {
@@ -938,6 +957,7 @@ func TestFindAllSubmatch(t *testing.T) {
 						}
 					}
 				}
+			}
 		})
 	}
 }