From 1520edad55b3d5f19bbc39cba5ae2991d3fdfc77 Mon Sep 17 00:00:00 2001
From: Aadhavan Srinivasan <aadhavan@twomorecents.org>
Date: Fri, 24 Jan 2025 15:50:36 -0500
Subject: [PATCH] Enforce the rule that character classes must have at least
 one character; interpret literal closing brackets as regular characters

---
 compile.go | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/compile.go b/compile.go
index bc967df..e3707ca 100644
--- a/compile.go
+++ b/compile.go
@@ -272,7 +272,7 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 		6. If current character is '{', find the appropriate numeric specifier (range start, range end). Apply the range to the postfixNode at the end of outQueue.
 		*/
 		c := re_postfix[i]
-		if isNormalChar(c) {
+		if isNormalChar(c) || isSpecialCharWithMetacharReplacement(c) {
 			if caseInsensitive {
 				outQueue = append(outQueue, newPostfixNode(allCases(c)...))
 			} else {
@@ -280,7 +280,7 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 			}
 			continue
 		}
-		// Escape character
+
 		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)
 				return nil, fmt.Errorf("ERROR: Backslash with no escape character.")
@@ -412,8 +412,9 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 			}
 		}
 		if c == LBRACKET { // Used for character classes
-			endOfRange := false // Set to 'true' when we encounter a CHAR_RANGE metacharacter
-			i++                 // Step forward so we can look at the character class
+			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
 			var invertMatch bool
 			if re_postfix[i] == '^' {
 				invertMatch = true
@@ -421,7 +422,7 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 			}
 			chars := make([]postfixNode, 0) // List of nodes - used only for character classes
 			for i < len(re_postfix) {
-				if re_postfix[i] == RBRACKET {
+				if firstCharAdded && re_postfix[i] == RBRACKET {
 					break
 				}
 				if re_postfix[i] == CHAR_RANGE {
@@ -481,9 +482,20 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 						i++
 					}
 				} else {
+					if !firstCharAdded && re_postfix[i] > 0xF0000 { // It's a metacharacter that I defined, I'll have to convert it back to the regular character before adding it back, because I haven't added any characters yet. For example, '[[]', the second LBRACKET should be treated like a literal bracket.
+						switch re_postfix[i] {
+						case LBRACKET:
+							chars = append(chars, newPostfixCharNode('['))
+						case RBRACKET:
+							chars = append(chars, newPostfixCharNode(']'))
+						default:
+							return nil, fmt.Errorf("Error parsing high-range unicode value in character class.")
+						}
+					}
 					chars = append(chars, newPostfixCharNode(re_postfix[i]))
 					i++
 				}
+				firstCharAdded = true
 
 				if endOfRange { // The previous character was an unescaped hyphen, which (in the context of a character class) means the character that was last appended is the end of a character range
 					// Things to note: