Loosened restrictions for concatenation - It's okay if one of the

elements is missing

compile.go

@@ -123,9 +123,12 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 		} else if c == '\\' && i < len(re_runes_orig)-1 && re_runes_orig[i+1] == '\\' { // Escaped backslash
 			re_runes = append(re_runes, ESC_BACKSLASH)
 			i++
-		} else if c == '[' && (i == 0 || re_runes_orig[i-1] != '\\')
-
-
+		} else if c == '[' && (i == 0 || re_runes[len(re_runes)-1] != '\\') {
+			re_runes = append(re_runes, LBRACKET)
+			continue
+		} else if c == ']' && (i == 0 || re_runes[len(re_runes)-1] != '\\') {
+			re_runes = append(re_runes, RBRACKET)
+			continue
 		} else {
 			re_runes = append(re_runes, c)
 		}
@@ -145,31 +148,28 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 	i := 0
 	for i < len(re_runes) {
 		re_postfix = append(re_postfix, re_runes[i])
-		if re_runes[i] == '[' && (i == 0 || re_runes[i-1] != '\\') { // We do not touch things inside brackets, unless they are escaped. Inside this block, the only task is to expand character ranges into their constituent characters.
-			re_postfix[len(re_postfix)-1] = LBRACKET         // Replace the '[' character with LBRACKET. This allows for easier parsing of all characters (including opening and closing brackets) within the character class
+		if re_runes[i] == LBRACKET && (i == 0 || re_runes[i-1] != '\\') { // We do not touch things inside brackets, unless they are escaped.
 			toAppend := make([]rune, 0) // Holds all the runes in the current character class
-			if i < len(re_runes)-1 && re_runes[i+1] == '^' { // Inverting class - match everything NOT in brackets
-				re_postfix = append(re_postfix, '^')
-				i++ // Skip opening bracket and caret
+
+			i++                     // Skip past LBRACKET, because it was already added
+			if i >= len(re_runes) { // Sanity check before we start
+				return nil, fmt.Errorf("Opening bracket without closing bracket.")
 			}
-			if i < len(re_runes)-1 && re_runes[i+1] == ']' { // Nothing inside brackets - panic.
-				return nil, fmt.Errorf("Empty character class.")
-			}
-			for re_runes[i] != ']' || i == 0 || re_runes[i-1] == '\\' {
-				i++ // Skip all characters inside _unescaped_ brackets (we are _not_ at a closing bracket, or if we are, the previous character is a backslash)
+
+			for re_runes[i] != RBRACKET || i == 0 || re_runes[i-1] == '\\' { // Skip all characters inside _unescaped_ brackets (we are _not_ at a closing bracket, or if we are, the previous character is a backslash)
 				// Make sure we haven't exceeded the length of the string. If we did, then the regex doesn't actually have a closing bracket and we should throw an error.
 				if i >= len(re_runes) {
 					return nil, fmt.Errorf("Opening bracket without closing bracket.")
 				}
 
-				if re_runes[i] == '-' && (i > 0 && re_runes[i-1] != '\\') && (i < len(re_runes)-1 && re_runes[i+1] != ']') { // Unescaped hyphen, that has some character (not a RBRACKET) after it - This represents a character range, so we replace with CHAR_RANGE. This metacharacter will be used later on to construct the range
+				if re_runes[i] == '-' && (i > 0 && re_runes[i-1] != '\\') && (i < len(re_runes)-1 && re_runes[i+1] != RBRACKET) { // Unescaped hyphen, that has some character (not a RBRACKET) after it - This represents a character range, so we replace with CHAR_RANGE. This metacharacter will be used later on to construct the range
 					re_runes[i] = CHAR_RANGE
 				}
-
 				toAppend = append(toAppend, re_runes[i])
+				i++
 			}
-			// Replace the last character (which should have been ']', with RBRACKET
-			toAppend[len(toAppend)-1] = RBRACKET
+			// Add in the RBRACKET
+			toAppend = append(toAppend, RBRACKET)
 			re_postfix = append(re_postfix, toAppend...)
 		}
 		if i < len(re_runes) && re_runes[i] == '{' && (i > 0 && re_runes[i-1] != '\\') { // We don't touch things inside braces, either
@@ -284,6 +284,17 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 			}
 			continue
 		}
+		// Since every unescaped bracket is replaced by a LBRACKET / RBRACKET, there may
+		// have been false positives. For example, the regex ']' has a closing bracket, but it
+		// isn't denoting a character class; it's just a regular character. Since it's not escaped,
+		// though, I would have converted this into an RBRACKET.
+		// To deal with this, I make the following assertion:
+		// 	If at any point I see an RBRACKET 'in the wild' (not in a character class), then it must be
+		// 	a regular character, with no special significance.
+		if c == RBRACKET {
+			outQueue = append(outQueue, newPostfixCharNode(']'))
+			continue
+		}
 
 		if c == '\\' { // Escape character - invert special and non-special characters eg. \( is treated as a literal parentheses, \b is treated as word boundary
 			if i == len(re_postfix)-1 { // End of string - panic, because backslash is an escape character (something needs to come after it)
@@ -419,6 +430,10 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 			firstCharAdded := false // A character class must have at least 1 character. This flag checks if the first character has been added.
 			endOfRange := false     // Set to 'true' when we encounter a CHAR_RANGE metacharacter
 			i++                     // Step forward so we can look at the character class
+			//  Oops, there's nothing there to look at
+			if i >= len(re_postfix) {
+				return nil, fmt.Errorf("Opening bracket with no closing bracket.")
+			}
 			var invertMatch bool
 			if re_postfix[i] == '^' {
 				invertMatch = true
@@ -643,6 +658,21 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 func thompson(re []postfixNode) (Reg, error) {
 	nfa := make([]*State, 0) // Stack of states
 	numGroups := 0           // Number of capturing groups
+
+	// If thompson() receives an empty regex, then whatever was given to shuntingYard()
+	// was parsed away. This doesn't mean that the regex itself is empty.
+	// For example, it could have been '(?:)'. This is an empty non-capturing group. Since
+	// shuntingYard() doesn't include non-capturing groups in its output (and the group contains
+	// nothing), the output of shuntingYard() (and the input to thompson()) ends up being empty.
+	// In these cases, we will return an NFA with 1 state, with an assertion that is always true.
+	if len(re) == 0 {
+		start := newState()
+		start.content = newContents(EPSILON)
+		start.isEmpty = true
+		start.assert = ALWAYS_TRUE
+		nfa = append(nfa, &start)
+	}
+
 	for _, c := range re {
 		if c.nodetype == CHARACTER || c.nodetype == ASSERTION {
 			state := State{}
@@ -749,15 +779,36 @@ func thompson(re []postfixNode) (Reg, error) {
 			// and then some other node.
 			// These three nodes (LPAREN, the middle node and RPAREN) are extracted together, concatenated
 			// and added back in.
+			// If the middle node doesn't exist (ie. something like '()' ), that's fine, I just connect the LPAREN
+			// and RPAREN nodes.
+			// If neither node exists, that's a problem so I return an error.
 			if c.nodetype == RPAREN {
 				s.groupEnd = true
-				middleNode := mustPop(&nfa)
-				lparenNode := mustPop(&nfa)
+				middleNode, err1 := pop(&nfa)
+				lparenNode, err2 := pop(&nfa)
+				if err1 != nil && err2 != nil {
+					return Reg{}, fmt.Errorf("Imbalanced parentheses.")
+				} else if err2 != nil { // There was no third node. ie. something like '()'
+					lparenNode = middleNode
+					if lparenNode.groupBegin != true { // There are only two nodes, but the first one isn't an LPAREN.
+						return Reg{}, fmt.Errorf("Imbalanced parentheses.")
+					}
 					s.groupNum = lparenNode.groupNum
+					to_add := concatenate(lparenNode, s)
+					nfa = append(nfa, to_add)
+				} else {
+					// At this point, we assume all three nodes are valid ('lparenNode', 'middleNode' and 's')
+					if lparenNode.groupBegin {
+						s.groupNum = lparenNode.groupNum
+					} else if middleNode.groupBegin { // Something like 'a()'
+						s.groupNum = middleNode.groupNum
+					} else { // A middleNode and lparenNode exist, but neither is actually an LPAREN.
+						return Reg{}, fmt.Errorf("Imbalanced parentheses.")
+					}
 					tmp := concatenate(lparenNode, middleNode)
 					to_add := concatenate(tmp, s)
 					nfa = append(nfa, to_add)
-
+				}
 			}
 		}
 		if c.nodetype == CHARCLASS { // A Character class consists of all the nodes in it, alternated
@@ -777,9 +828,16 @@ func thompson(re []postfixNode) (Reg, error) {
 		switch c.nodetype {
 		case CONCATENATE:
 			s2 := mustPop(&nfa)
-			s1 := mustPop(&nfa)
+			// Relax the requirements for concatenation a little bit - If
+			// the second element is not found ie. the postfixNodes look
+			// like 'a~', then that's fine, we just skip the concatenation.
+			s1, err := pop(&nfa)
+			if err != nil {
+				nfa = append(nfa, s2)
+			} else {
 				s1 = concatenate(s1, s2)
 				nfa = append(nfa, s1)
+			}
 		case KLEENE: // Create a 0-state, concat the popped state after it, concat the 0-state after the popped state
 			s1 := mustPop(&nfa)
 			stateToAdd := kleene(*s1)

nfa.go

@@ -18,6 +18,7 @@ const (
 	NLA         // Negative lookahead
 	PLB         // Positive lookbehind
 	NLB         // Negative lookbehind
+	ALWAYS_TRUE // An assertion that is always true
 )
 
 type State struct {
@@ -103,6 +104,9 @@ func cloneStateHelper(state *State, cloneMap map[*State]*State) *State {
 // Checks if the given state's assertion is true. Returns true if the given
 // state doesn't have an assertion.
 func (s State) checkAssertion(str []rune, idx int) bool {
+	if s.assert == ALWAYS_TRUE {
+		return true
+	}
 	if s.assert == SOS {
 		return idx == 0
 	}

re_test.go

@@ -237,6 +237,34 @@ var reTests = []struct {
 	{`(abc`, nil, `-`, nil},
 	{`a]`, nil, `a]`, []Group{{0, 2}}},
 	{`a[]]b`, nil, `a]b`, []Group{{0, 3}}},
+	{`a[\]]b`, nil, `a]b`, []Group{{0, 3}}},
+	{`a[^bc]d`, nil, `aed`, []Group{{0, 3}}},
+	{`a[^bc]d`, nil, `abd`, []Group{}},
+	{`a[^-b]c`, nil, `adc`, []Group{{0, 3}}},
+	{`a[^-b]c`, nil, `a-c`, []Group{}},
+	{`a[^]b]c`, nil, `a]c`, []Group{}},
+	{`a[^]b]c`, nil, `adc`, []Group{{0, 3}}},
+	{`\ba\b`, nil, `a-`, []Group{{0, 1}}},
+	{`\ba\b`, nil, `-a`, []Group{{1, 2}}},
+	{`\ba\b`, nil, `-a-`, []Group{{1, 2}}},
+	{`\by\b`, nil, `xy`, []Group{}},
+	{`\by\b`, nil, `yz`, []Group{}},
+	{`\by\b`, nil, `xyz`, []Group{}},
+	{`x\b`, nil, `xyz`, []Group{}},
+	{`x\B`, nil, `xyz`, []Group{{0, 1}}},
+	{`\Bz`, nil, `xyz`, []Group{{2, 3}}},
+	{`z\B`, nil, `xyz`, []Group{}},
+	{`\Bx`, nil, `xyz`, []Group{}},
+	{`\Ba\B`, nil, `a-`, []Group{}},
+	{`\Ba\B`, nil, `-a`, []Group{}},
+	{`\Ba\B`, nil, `-a-`, []Group{}},
+	{`\By\B`, nil, `xy`, []Group{}},
+	{`\By\B`, nil, `yz`, []Group{}},
+	{`\By\b`, nil, `xy`, []Group{{1, 2}}},
+	{`\by\B`, nil, `yz`, []Group{{0, 1}}},
+	{`\By\B`, nil, `xyz`, []Group{{1, 2}}},
+	{`ab|cd`, nil, `abc`, []Group{{0, 2}}},
+	{`ab|cd`, nil, `abcd`, []Group{{0, 2}, {2, 4}}},
 
 	// Todo - add numeric range tests
 }
@@ -267,6 +295,9 @@ var groupTests = []struct {
 	{"(a?)a?", nil, "aa", []Match{[]Group{{0, 2}, {0, 1}}, []Group{{2, 2}, {2, 2}}}},
 	{"a((b.d){3})", nil, "abfdbhdbid", []Match{[]Group{{0, 10}, {1, 10}, {7, 10}}}},
 	{`(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)\071`, nil, `abcdefghijkl9`, []Match{[]Group{{0, 13}, {0, 1}, {1, 2}, {2, 3}, {3, 4}, {4, 5}, {5, 6}, {6, 7}, {7, 8}, {8, 9}, {9, 10}, {10, 11}, {11, 12}}}},
+	{`()ef`, nil, `def`, []Match{[]Group{{1, 3}, {1, 1}}}},
+	{`(?:)ef`, nil, `def`, []Match{[]Group{{1, 3}}}},
+	{`(?:)`, nil, `def`, []Match{[]Group{{0, 0}}, []Group{{1, 1}}, []Group{{2, 2}}, []Group{{3, 3}}}},
 }
 
 func TestFindAllMatches(t *testing.T) {