From 1a7fd12569e4a1879674845b067a63a288c363eb Mon Sep 17 00:00:00 2001
From: Aadhavan Srinivasan <aadhavan@twomorecents.org>
Date: Tue, 29 Oct 2024 10:05:14 -0400
Subject: [PATCH] Added support for some escaped metacharacters

---
 main.go | 44 +++++++++++++++++++++++++++++++++-----------
 1 file changed, 33 insertions(+), 11 deletions(-)

diff --git a/main.go b/main.go
index daeef74..5b82e03 100644
--- a/main.go
+++ b/main.go
@@ -35,7 +35,7 @@ func shuntingYard(re string) []postfixNode {
 	re_runes := []rune(re) // Convert the string to a slice of runes to allow iteration through it
 	/* 	Add concatenation operators.
 	Only add a concatenation operator between two characters if both the following conditions are met:
-		1. 	The first character isn't an opening parantheses or alteration operator.
+		1. 	The first character isn't an opening parantheses or alteration operator (or an escape character)
 			a. This makes sense, because these operators can't be _concatenated_ with anything else.
 		2. The second character isn't a 'closing operator' - one that applies to something before it
 			a. Again, these operators can'be concatenated _to_. They can, however, be concatenated _from_.
@@ -67,7 +67,7 @@ func shuntingYard(re string) []postfixNode {
 			}
 			continue
 		}
-		if re_runes[i] != '(' && re_runes[i] != '|' {
+		if re_runes[i] != '(' && re_runes[i] != '|' && re_runes[i] != '\\' {
 			if i < len(re_runes)-1 {
 				if re_runes[i+1] != '|' && re_runes[i+1] != '*' && re_runes[i+1] != '+' && re_runes[i+1] != '?' && re_runes[i+1] != ')' {
 					re_postfix = append(re_postfix, CONCAT)
@@ -98,16 +98,39 @@ func shuntingYard(re string) []postfixNode {
 			outQueue = append(outQueue, newPostfixNode(c))
 			continue
 		}
-		// Escape character - NOT IMPLEMENTED YET - DO NOT USE
-		// if c == '\\' { // Escape character - next character is treated as alphanum
-		// 	if i == len(re_postfix)-1 { // End of string - panic, because backslash is an escape character (something needs to come after it)
-		// 		panic("ERROR: Backslash with no escape character.")
-		// 	}
-		// 	outQueue = append(outQueue, re_postfix[i+1])
-		// }
+		// 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)
+				panic("ERROR: Backslash with no escape character.")
+			}
+			i++
+			switch re_postfix[i] {
+			case 's': // Whitespace
+				outQueue = append(outQueue, newPostfixNode(whitespaceChars...))
+			case 'S': // Non-whitespace - I am doing this in a fancy way, generating all dot characters, then removing whitespace characters from it
+				outQueue = append(outQueue, newPostfixNode(slices.DeleteFunc(dotChars(), func(r rune) bool {
+					return !slices.Contains(whitespaceChars, r)
+				})...))
+			case 'd': // Digits
+				outQueue = append(outQueue, newPostfixNode(digitChars...))
+			case 'D': // Non-digits - same fancy way as 'S'
+				outQueue = append(outQueue, newPostfixNode(slices.DeleteFunc(dotChars(), func(r rune) bool {
+					return !slices.Contains(digitChars, r)
+				})...))
+			case 'w': // word character
+				outQueue = append(outQueue, newPostfixNode(wordChars...))
+			case 'W': // Non-word character - same fancy way as 'S' and 'D'
+				outQueue = append(outQueue, newPostfixNode(slices.DeleteFunc(dotChars(), func(r rune) bool {
+					return !slices.Contains(wordChars, r)
+				})...))
+			default: // None of the above - append it as a regular character
+				outQueue = append(outQueue, newPostfixCharNode(re_postfix[i]))
+			}
+			continue // Escaped character will automatically be skipped when loop variable increments
+		}
 
 		if c == '.' { // Dot metacharacter - represents 'any' character, but I am only adding Unicode 0020-007E
-			outQueue = append(outQueue, newPostfixNode(dotCharacters()...))
+			outQueue = append(outQueue, newPostfixNode(dotChars()...))
 			continue
 		}
 		if isOperator(c) {
@@ -247,7 +270,6 @@ func main() {
 	if err != nil {
 		panic(err)
 	}
-
 	fmt.Scanln(&test_str)
 	re_postfix := shuntingYard(re)
 	// fmt.Println(re_postfix)