regex/compile.go

package main

import (
	"fmt"
	"slices"
	"strconv"
	"unicode"
)

// Holds a list of all characters that are _not_ matched by the dot metacharacter
var notDotChars []rune

// A Reg represents the result of compiling a regular expression. It contains
// the startState of the NFA representation of the regex, and the number of capturing
// groups in the regex.
type Reg struct {
	start     *State
	numGroups int
}

const CONCAT rune = '~'

// Flags for shuntingYard - control its behavior
type ReFlag int

const (
	RE_NO_FLAGS ReFlag = iota
	RE_CASE_INSENSITIVE
	RE_MULTILINE
)

func isOperator(c rune) bool {
	if c == '+' || c == '?' || c == '*' || c == '|' || c == CONCAT {
		return true
	}
	return false
}

/* priority returns the priority of the given operator */
func priority(op rune) int {
	precedence := []rune{'|', CONCAT, '+', '*', '?'}
	return slices.Index(precedence, op)
}

/*
The Shunting-Yard algorithm is used to convert the given infix (regeular) expression to postfix.
The primary benefit of this is getting rid of parentheses.
It also inserts explicit concatenation operators to make parsing easier in Thompson's algorithm.
An error can be returned for a multitude of reasons - the reason is specified in the error string.
The function also takes in 0 or more flags, which control the behavior of the parser.
See: https://blog.cernera.me/converting-regular-expressions-to-postfix-notation-with-the-shunting-yard-algorithm/
*/
func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
	// Check which flags are enabled

	caseInsensitive := false
	// In Multiline mode, the newline character is considered a
	// 'dot' character ie. the dot metacharacter matches a newline as well.
	if slices.Contains(flags, RE_MULTILINE) {
		notDotChars = []rune{}
	} else {
		notDotChars = []rune{'\n'}
	}
	if slices.Contains(flags, RE_CASE_INSENSITIVE) {
		caseInsensitive = true
	}

	re_postfix := make([]rune, 0)
	// Convert the string to a slice of runes to allow iteration through it
	re_runes_orig := []rune(re) // This is the rune slice before the first parsing loop (which detects and replaces numeric ranges)
	re_runes := make([]rune, 0)
	// Check for numeric range. If we are at the start of a numeric range,
	// skip to end and construct the equivalent regex for the range.
	// The reason this is outside the loop below, is that it actually modifies
	// the given regex (we 'cut' the numeric range and 'paste' an equivalent regex).
	// It also makes the overall parsing easier, since I don't have to worry about the numeric range
	// anymore.
	// Eventually, I might be able to add it into the main parsing loop, to reduce the time
	// complexity.
	// A numeric range has the syntax: <num1-num2>. Ir matches all numbers in this range.
	//
	// Also check for non-capturing groups. The LPAREN of a non-capturing group looks like this: '(?:'
	// I take this out, and put in a special character - NONCAPLPAREN_CHAR.
	//
	// Finally, check for escaped backslashes. Replace these with the BACKSLASH metacharacter. Later, in thompson(),
	// these will be converted back. This avoids confusiuon in detecting whether a character is escaped eg. detecting
	// whether '\\[a]' has an escaped opening bracket (it doesn't).
	for i := 0; i < len(re_runes_orig); i++ {
		c := re_runes_orig[i]
		if c == '<' && (i == 0 || (re_runes_orig[i-1] != '\\' && re_runes_orig[i-1] != '?')) {
			i++ // Step over opening angle bracket
			tmpStr := ""
			hyphenFound := false
			for i < len(re_runes_orig) && re_runes_orig[i] != '>' {
				if !unicode.IsDigit(re_runes_orig[i]) {
					if re_runes_orig[i] != '-' || (hyphenFound) {
						return nil, fmt.Errorf("Invalid numeric range.")
					}
				}
				if re_runes_orig[i] == '-' {
					hyphenFound = true
				}
				tmpStr += string(re_runes_orig[i])
				i++
			}
			// End of string reached and last character doesn't close the range
			if i == len(re_runes_orig) && re_runes_orig[len(re_runes_orig)-1] != '>' {
				return nil, fmt.Errorf("Numeric range not closed.")
			}
			if len(tmpStr) == 0 {
				return nil, fmt.Errorf("Empty numeric range.")
			}
			// Closing bracket will be skipped when the loop variable increments
			var rangeStart int
			var rangeEnd int
			fmt.Sscanf(tmpStr, "%d-%d", &rangeStart, &rangeEnd)
			regex := range2regex(rangeStart, rangeEnd)
			re_runes = append(re_runes, []rune(regex)...)
		} else if c == '(' && i < len(re_runes_orig)-2 && re_runes_orig[i+1] == '?' && re_runes_orig[i+2] == ':' {
			re_runes = append(re_runes, NONCAPLPAREN_CHAR)
			i += 2
		} 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 {
			re_runes = append(re_runes, c)
		}
	}

	/* 	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 (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_.
	Caveats:
		1. Don't mess with anything inside brackets - character class
		2. Don't mess with anything inside braces - numeric repetition
		3. Don't mess with any lookarounds.
	*/
	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
			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
			}
			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)
				// TODO: Check for escaped characters

				// Check ahead for character range
				if i < len(re_runes)-2 && re_runes[i+1] == '-' {
					rangeStart := re_runes[i]
					rangeEnd := re_runes[i+2]
					if int(rangeEnd) < int(rangeStart) {
						return nil, fmt.Errorf("Range is out of order.")
					}

					for i := rangeStart; i <= rangeEnd; i++ {
						toAppend = append(toAppend, i)
					}

					i += 2 // Skip start and hyphen (end will automatically be skipped on next iteration of loop)
					continue
				}
				toAppend = append(toAppend, re_runes[i])
			}
			// Replace the last character (which should have been ']', with RBRACKET
			toAppend[len(toAppend)-1] = 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
			i++ // Skip opening brace
			for i < len(re_runes) && re_runes[i] != '}' {
				re_postfix = append(re_postfix, re_runes[i])
				i++
			}
			if i == len(re_runes) {
				return nil, fmt.Errorf("Invalid numeric specifier.")
			}
			re_postfix = append(re_postfix, re_runes[i]) // Append closing brace
		}
		if i < len(re_runes)-3 && string(re_runes[i+1:i+4]) == "(?:" { // Non-capturing lparen
			re_postfix = append(re_postfix, NONCAPLPAREN_CHAR)
			i += 3
		}
		if i < len(re_runes) && re_runes[i] == '\\' { // Something is being escaped (I don't add the backslash to re_postfix, because it was already added earlier)
			i++
			if i >= len(re_runes) {
				return nil, fmt.Errorf("Stray backslash in expression.")
			}
			if re_runes[i] == 'x' {
				re_postfix = append(re_postfix, re_runes[i])
				i++
				if i >= len(re_runes) {
					return nil, fmt.Errorf("Stray backslash in expression.")
				}
				if re_runes[i] == '{' {
					re_postfix = append(re_postfix, re_runes[i:i+8]...)
					i += 7
					if i >= len(re_runes) {
						return nil, fmt.Errorf("Stray backslash in expression.")
					}
				} else if isHex(re_runes[i]) {
					re_postfix = append(re_postfix, re_runes[i:i+2]...)
					i += 2
				} else {
					return nil, fmt.Errorf("Invalid hex value in expression.")
				}
			} else if isOctal(re_runes[i]) {
				numDigits := 1
				for i+numDigits < len(re_runes) && numDigits < 3 && isOctal(re_runes[i+numDigits]) { // Skip while we see an octal character (max of 3)
					numDigits++
				}
				re_postfix = append(re_postfix, re_runes[i:i+numDigits]...)
				i += (numDigits - 1) // I have to move back a step, so that I can add a concatenation operator if necessary, and so that the increment at the bottom of the loop works as intended
			} else {
				re_postfix = append(re_postfix, re_runes[i])
			}
		}
		if i < len(re_runes) && re_runes[i] == '(' && (i == 0 || re_runes[i-1] != '\\') && (i < len(re_runes)-2 && re_runes[i+1] == '?' && slices.Contains([]rune{'=', '!', '<'}, re_runes[i+2])) { // Unescaped open parentheses followed by question mark then '<', '!' or '=' => lokaround. Don't mess with it.
			i++ // Step inside
			if i == len(re_runes)-1 || (re_runes[i+1] != '=' && re_runes[i+1] != '!' && re_runes[i+1] != '<') {
				return nil, fmt.Errorf("Invalid regex. Lookaround intended?")
			}
			re_postfix = append(re_postfix, re_runes[i])
			i++
			numOpenParens := 1
			for numOpenParens != 0 {
				if i >= len(re_runes) {
					return nil, fmt.Errorf("Unclosed lookaround.")
				}
				if re_runes[i] == '(' || re_runes[i] == NONCAPLPAREN_CHAR {
					numOpenParens++
				}
				if re_runes[i] == ')' {
					numOpenParens--
					if numOpenParens == 0 {
						break
					}
				}
				re_postfix = append(re_postfix, re_runes[i])
				i++
			}
			continue
		}
		if i < len(re_runes) && (re_runes[i] != '(' && re_runes[i] != NONCAPLPAREN_CHAR && re_runes[i] != '|' && re_runes[i] != '\\') || (i > 0 && re_runes[i-1] == '\\') { // Every character should be concatenated if it is escaped
			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_runes[i+1] != '{' {
					re_postfix = append(re_postfix, CONCAT)
				}
			}
		}
		i++
	}

	opStack := make([]rune, 0)         // Operator stack
	outQueue := make([]postfixNode, 0) // Output queue

	// Actual algorithm
	numOpenParens := 0 // Number of open parentheses
	for i := 0; i < len(re_postfix); i++ {
		/* Two cases:
		1. Current character is alphanumeric - send to output queue
		2. Current character is operator - do the following:
			a. If current character has greater priority than top of opStack, push to opStack.
			b. If not, keep popping from opStack (and appending to outQueue) until:
				i. opStack is empty, OR
				ii. current character has greater priority than top of opStack
		3. If current character is '(' or NONCAPLPAREN_CHAR, push to opStack
		4. If current character is ')', pop from opStack (and append to outQueue) until '(' is found. Discard parantheses.
		5. If current character is '[', find all the characters until ']', then create a postfixNode containing all these contents. Add this node to outQueue.
		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 caseInsensitive {
				outQueue = append(outQueue, newPostfixNode(allCases(c)...))
			} else {
				outQueue = append(outQueue, newPostfixNode(c))
			}
			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.")
			}
			i++
			if re_postfix[i] == 'x' { // Hex value
				i++
				if re_postfix[i] == '{' && i < len(re_postfix)-6 { // Expanded hex code
					var hexVal int
					n, err := fmt.Sscanf(string(re_postfix[i:]), "{%x}", &hexVal)
					if n < 1 || err != nil {
						return nil, fmt.Errorf("Error parsing expanded hex code in expression.")
					}
					outQueue = append(outQueue, newPostfixCharNode(rune(hexVal)))
					i += 7
				} else if i < len(re_postfix)-1 { // Two-digit hex code
					hexVal, err := strconv.ParseInt(string([]rune{re_postfix[i], re_postfix[i+1]}), 16, 64) // Convert the two hex values into a rune slice, then to a string. Parse the string into an int with strconv.ParseInt()
					if err != nil {
						return nil, fmt.Errorf("Error parsing hex characters in expression.")
					}
					i++ // Loop increment will take care of going forward
					outQueue = append(outQueue, newPostfixCharNode(rune(hexVal)))
				} else {
					return nil, fmt.Errorf("Not enough hex characters found in expression.")
				}
			} else if isOctal(re_postfix[i]) { // Octal value
				var octVal int64
				var octValStr string
				numDigitsParsed := 0
				for (i+numDigitsParsed) < len(re_postfix) && isOctal(re_postfix[i+numDigitsParsed]) && numDigitsParsed <= 3 {
					octValStr += string(re_postfix[i+numDigitsParsed])
					numDigitsParsed++
				}
				octVal, err := strconv.ParseInt(octValStr, 8, 32)
				if err != nil {
					return nil, fmt.Errorf("Error parsing octal value in expression.")
				}
				if octVal > 0777 {
					return nil, fmt.Errorf("Invalid octal value in expression.")
				}
				i += numDigitsParsed - 1 // Shift forward by the number of digits that were parsed. Move back one character, because the loop increment will move us back to the next character automatically
				outQueue = append(outQueue, newPostfixCharNode(rune(octVal)))
			} else {
				escapedNode, err := newEscapedNode(re_postfix[i], false)
				if err != nil {
					return nil, fmt.Errorf("Invalid escape character in expression.")
				}
				outQueue = append(outQueue, escapedNode)
			}
			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, newPostfixDotNode())
			continue
		}
		if c == '^' { // Start-of-string assertion
			outQueue = append(outQueue, newPostfixNode(c))
		}
		if c == '$' { // End-of-string assertion
			outQueue = append(outQueue, newPostfixNode(c))
		}
		// Check if we're at the start of a lookaround
		if c == '(' && i < len(re_postfix)-1 && re_postfix[i+1] == '?' {
			i += 2      // Skip opening paren and question mark
			regex := "" // Stores lookaround regex
			numOpenParens := 1
			for numOpenParens != 0 {
				if i >= len(re_postfix) {
					return nil, fmt.Errorf("Unclosed lookaround.")
				}
				if re_postfix[i] == '(' || re_postfix[i] == NONCAPLPAREN_CHAR {
					numOpenParens++
				}
				if re_postfix[i] == ')' {
					numOpenParens--
					if numOpenParens == 0 {
						break
					}
				}
				regex += string(re_postfix[i])
				i++
			}
			if len(regex) <= 1 { // Nothing in regex - panic
				return nil, fmt.Errorf("Invalid lookaround. (too short?)")
			}
			// 'regex' should now contain the lookaround regex, plus the characters at the start (which indicate pos/neg, ahead/behind)
			// Now we should filter that out.
			toAppend := postfixNode{nodetype: ASSERTION, startReps: 1, endReps: 1}
			if regex[0] == '<' { // Lookbehind
				toAppend.lookaroundDir = LOOKBEHIND
				regex = regex[1:]
			} else if regex[0] == '=' || regex[0] == '!' {
				toAppend.lookaroundDir = LOOKAHEAD
			} else {
				return nil, fmt.Errorf("Invalid lookaround.")
			}
			// Positive or negative
			if regex[0] == '=' { // Positive
				toAppend.lookaroundSign = POSITIVE
				toAppend.contents = []rune(regex[1:])
			} else if regex[0] == '!' { // Negative
				toAppend.lookaroundSign = NEGATIVE
				toAppend.contents = []rune(regex[1:])
			} else {
				return nil, fmt.Errorf("Invalid lookaround.")
			}
			outQueue = append(outQueue, toAppend)
			continue
		}
		if isOperator(c) {
			if len(opStack) == 0 {
				opStack = append(opStack, c)
			} else {
				topStack, err := peek(opStack)
				if err != nil {
					return nil, fmt.Errorf("Operator without operand.")
				}
				if priority(c) > priority(topStack) { // 2a
					opStack = append(opStack, c)
				} else {
					for priority(c) <= priority(topStack) { // 2b
						to_append := mustPop(&opStack)
						outQueue = append(outQueue, newPostfixNode(to_append))
						topStack, _ = peek(opStack)
					}
					opStack = append(opStack, c)
				}
			}
		}
		if c == LBRACKET { // Used for character classes
			i++ // Step forward so we can look at the character class
			var invertMatch bool
			if re_postfix[i] == '^' {
				invertMatch = true
				i++
			}
			chars := make([]postfixNode, 0) // List of nodes - used only for character classes
			for i < len(re_postfix) {
				if re_postfix[i] == RBRACKET {
					break
				}
				if re_postfix[i] == '\\' { // Backslash indicates a character to be escaped
					if i == len(re_postfix)-1 {
						return nil, fmt.Errorf("Stray backslash in character class.")
					}
					i++ // Step past backslash

					if re_postfix[i] == 'x' { // Hex value
						i++
						if re_postfix[i] == '{' && i < len(re_postfix)-7 { // Expanded hex code
							var hexVal int
							n, err := fmt.Sscanf(string(re_postfix[i:]), "{%x}", &hexVal)
							if n < 1 || err != nil {
								return nil, fmt.Errorf("Error parsing expanded hex code in character class.")
							}
							chars = append(chars, newPostfixCharNode(rune(hexVal)))
							i += 8
						} else if i < len(re_postfix)-2 { // Two-digit hex code
							hexVal, err := strconv.ParseInt(string([]rune{re_postfix[i], re_postfix[i+1]}), 16, 64) // Convert the two hex values into a rune slice, then to a string. Parse the string into an int with strconv.ParseInt()
							if err != nil {
								return nil, fmt.Errorf("Error parsing hex characters in character class.")
							}
							i += 2
							chars = append(chars, newPostfixCharNode(rune(hexVal)))
						} else {
							return nil, fmt.Errorf("Not enough hex characters found in character class.")
						}
					} else if isOctal(re_postfix[i]) { // Octal value
						var octVal int64
						var octValStr string
						numDigitsParsed := 0
						for (i+numDigitsParsed) < len(re_postfix)-1 && isOctal(re_postfix[i+numDigitsParsed]) && numDigitsParsed <= 3 { // The '-1' exists, because even in the worst case (the character class extends till the end), the last character must be a closing bracket (and nothing else)
							octValStr += string(re_postfix[i+numDigitsParsed])
							numDigitsParsed++
						}
						octVal, err := strconv.ParseInt(octValStr, 8, 32)
						if err != nil {
							return nil, fmt.Errorf("Error parsing octal value in character class.")
						}
						if octVal > 0777 {
							return nil, fmt.Errorf("Invalid octal value in character class.")
						}
						i += numDigitsParsed // Shift forward by the number of characters parsed
						chars = append(chars, newPostfixCharNode(rune(octVal)))
					} else {
						escapedNode, err := newEscapedNode(re_postfix[i], true)
						if err != nil {
							return nil, fmt.Errorf("Invalid escape character in character class.")
						}
						chars = append(chars, escapedNode)
						i++
					}
				} else {
					chars = append(chars, newPostfixCharNode(re_postfix[i]))
					i++
				}
			}
			if i == len(re_postfix) { // We have reached the end of the string, so we didn't encounter a closing brakcet. Panic.
				return nil, fmt.Errorf("Opening bracket without closing bracket.")
			}
			outQueue = append(outQueue, newCharClassNode(chars, invertMatch))
			continue
		}
		if c == '{' {
			i++ // Skip opening brace
			// Three possibilities:
			// 1. Single number - {5}
			// 2. Range - {3,5}
			// 3. Start with no end, {3,}
			startRange := make([]rune, 0)
			startRangeNum := 0
			endRange := make([]rune, 0)
			endRangeNum := 0
			for i < len(re_postfix) && unicode.IsDigit(re_postfix[i]) {
				startRange = append(startRange, re_postfix[i])
				i++
			}
			if len(startRange) == 0 { // {} is not valid, neither is {,5}
				return nil, fmt.Errorf("Invalid numeric specifier.")
			}
			if i == len(re_postfix) {
				return nil, fmt.Errorf("Brace not closed.")
			}

			startRangeNum, err := strconv.Atoi(string(startRange))
			if err != nil {
				panic(err)
			}

			if re_postfix[i] == '}' { // Case 1 above
				endRangeNum = startRangeNum
			} else {
				if re_postfix[i] != ',' {
					return nil, fmt.Errorf("Invalid numeric specifier.")
				}
				i++ // Skip comma
				for i < len(re_postfix) && unicode.IsDigit(re_postfix[i]) {
					endRange = append(endRange, re_postfix[i])
					i++
				}
				if i == len(re_postfix) {
					return nil, fmt.Errorf("Brace not closed.")
				}
				if re_postfix[i] != '}' {
					return nil, fmt.Errorf("Invalid numeric specifier.")
				}
				if len(endRange) == 0 { // Case 3 above
					endRangeNum = INFINITE_REPS
				} else { // Case 2 above
					var err error
					endRangeNum, err = strconv.Atoi(string(endRange))
					if err != nil {
						panic(err)
					}
				}
			}

			idx := len(outQueue) - 1
			// Get the last added node
			if idx < 0 || outQueue[idx].nodetype == LPAREN {
				return nil, fmt.Errorf("Numeric specifier with no content.")
			}
			outQueue[idx].startReps = startRangeNum
			outQueue[idx].endReps = endRangeNum
		}
		if c == '(' || c == NONCAPLPAREN_CHAR {
			opStack = append(opStack, c)
			if c == '(' { // We only push _capturing_ group parentheses to outQueue
				outQueue = append(outQueue, newPostfixNode(c))
			}
			numOpenParens++
		}
		if c == ')' {
			// Keep popping from opStack until we encounter an opening parantheses or a NONCAPLPAREN_CHAR. Panic if we reach the end of the stack.
			var val rune
			var err error
			for val, err = peek(opStack); val != '(' && val != NONCAPLPAREN_CHAR; val, err = peek(opStack) {
				if err != nil {
					return nil, fmt.Errorf("Imbalanced parantheses.")
				}
				to_append := mustPop(&opStack)
				outQueue = append(outQueue, newPostfixNode(to_append))
			}
			_ = mustPop(&opStack) // Get rid of opening parentheses
			if val == '(' {       // Whatever was inside the parentheses was a _capturing_ group, so we append the closing parentheses as well
				outQueue = append(outQueue, newPostfixNode(')')) // Add closing parentheses
			}
			numOpenParens--
		}
	}

	// Pop all remaining operators (and append to outQueue)
	for len(opStack) > 0 {
		to_append := mustPop(&opStack)
		outQueue = append(outQueue, newPostfixNode(to_append))
	}

	if numOpenParens != 0 {
		return nil, fmt.Errorf("Imbalanced parantheses.")
	}

	return outQueue, nil
}

// Thompson's algorithm. Constructs Finite-State Automaton from given string.
// Returns start state and number of groups in regex.
func thompson(re []postfixNode) (Reg, error) {
	nfa := make([]*State, 0) // Stack of states
	numGroups := 0           // Number of capturing groups
	for _, c := range re {
		if c.nodetype == CHARACTER || c.nodetype == ASSERTION {
			state := State{}
			state.transitions = make(map[int][]*State)
			if c.allChars {
				state.allChars = true
				if len(c.except) != 0 {
					// For each node that I am 'excepting' (eg. in an inverted character class):
					// 		- If the node itself has exceptions, then the exceptions cancel out.
					// 		Eg. [^\w] == [\W]
					// 		- Since an allChars node is the only kind that _can_ have exceptions, that's what I check for.
					//		- If the node doesn't have exceptions (allChars == false) then the contents of the node are added to the except list.
					for _, node := range c.except {
						if node.allChars {
							state.allChars = false
							// For each postfixNode in node.except, extract the contents of the postfixNode. Concatenate them all,
							// and them to the state's _content_. As mentioned above, if the exception has exceptions, then we can match
							// those.
							nodeExceptChars := slices.Concat(Map(node.except, func(node postfixNode) []rune {
								return node.contents
							})...)
							state.content = rune2Contents(nodeExceptChars)
						} else {
							state.except = append(state.except, node.contents...)
						}
					}
				}
			}
			// Convert the current contents to []int, convert the result of rune2contents to []int, append then
			// convert back to stateContents.
			state.content = stateContents(append([]int(state.content), []int(rune2Contents(c.contents))...))
			state.output = make([]*State, 0)
			state.output = append(state.output, &state)
			state.isEmpty = false
			if c.nodetype == ASSERTION {
				state.isEmpty = true                 // This is a little weird. A lookaround has the 'isEmpty' flag set, even though it _isn't_ empty (the contents are the regex). But, there's so much error-checking that relies on this flag that it's better to keep it this way.
				state.content = newContents(EPSILON) // Ideally, an assertion shouldn't have any content, since it doesn't say anything about the content of string
				if c.lookaroundDir == 0 || c.lookaroundSign == 0 {
					switch c.contents[0] {
					case '^':
						state.assert = SOS
					case '$':
						state.assert = EOS
					case 'b':
						state.assert = WBOUND
					case 'B':
						state.assert = NONWBOUND
					}
				} else { // Lookaround
					state.lookaroundRegex = string(c.contents)
					if c.lookaroundDir == LOOKAHEAD {
						if c.lookaroundSign == POSITIVE {
							state.assert = PLA
						}
						if c.lookaroundSign == NEGATIVE {
							state.assert = NLA
						}
					}
					if c.lookaroundDir == LOOKBEHIND {
						if c.lookaroundSign == POSITIVE {
							state.assert = PLB
						}
						if c.lookaroundSign == NEGATIVE {
							state.assert = NLB
						}
					}
					tmpRe, err := shuntingYard(state.lookaroundRegex)
					if err != nil {
						return Reg{}, fmt.Errorf("Error parsing lookaround: %w", err)
					}
					reg, err := thompson(tmpRe)
					if err != nil {
						return Reg{}, fmt.Errorf("Error compiling lookaround: %w", err)
					}
					state.lookaroundNFA = reg.start
					state.lookaroundNumCaptureGroups = reg.numGroups

				}
			}

			// Replace ESC_BACKSLASH with actual backslash, so that we can actually check if we encounter it
			replaceByValue([]int(state.content), int(ESC_BACKSLASH), '\\')
			// Uncommenting this seems to make one of the test cases fail. Why?
			//			replaceByValue(state.except, ESC_BACKSLASH, '\\')

			nfa = append(nfa, &state)
		}
		if c.nodetype == LPAREN || c.nodetype == RPAREN {
			s := &State{}
			s.assert = NONE
			s.content = newContents(EPSILON)
			s.isEmpty = true
			s.output = make([]*State, 0)
			s.output = append(s.output, s)
			s.transitions = make(map[int][]*State)
			// LPAREN nodes are just added normally
			if c.nodetype == LPAREN {
				numGroups++
				s.groupBegin = true
				s.groupNum = numGroups
				nfa = append(nfa, s)
				continue
			}
			// For RPAREN nodes, I assume that the last two nodes in the list are an LPAREN,
			// and then some other node.
			// These three nodes (LPAREN, the middle node and RPAREN) are extracted together, concatenated
			// and added back in.
			if c.nodetype == RPAREN {
				s.groupEnd = true
				middleNode := mustPop(&nfa)
				lparenNode := mustPop(&nfa)
				s.groupNum = lparenNode.groupNum
				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
			// Map the list of nodes to a list of states, each state containing the contents of a specific node
			states := Map(c.nodeContents, func(node postfixNode) *State {
				s := newState()
				s.content = rune2Contents(node.contents)
				return &s
			})
			// Reduce the list of states down to a single state by alternating them
			toAdd := Reduce(states, func(s1 *State, s2 *State) *State {
				return alternate(s1, s2)
			})
			nfa = append(nfa, toAdd)
		}
		// Must be an operator if it isn't a character
		switch c.nodetype {
		case CONCATENATE:
			s2 := mustPop(&nfa)
			s1 := mustPop(&nfa)
			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 = append(nfa, stateToAdd)
		case PLUS: // a+ is equivalent to aa*
			s1 := mustPop(&nfa)
			s2 := kleene(*s1)
			s1 = concatenate(s1, s2)
			nfa = append(nfa, s1)
		case QUESTION: // ab? is equivalent to a(b|)
			s1 := mustPop(&nfa)
			s2 := question(s1)
			nfa = append(nfa, s2)
		case PIPE:
			s1 := mustPop(&nfa)
			s2 := mustPop(&nfa)
			s3 := alternate(s1, s2)
			nfa = append(nfa, s3)
		}
		if c.startReps != 1 || c.endReps != 1 { // Must have a numeric specifier attached to it
			if c.endReps != -1 && c.endReps < c.startReps {
				return Reg{}, fmt.Errorf("Numeric specifier - start greater than end.")
			}
			state := mustPop(&nfa)
			var stateToAdd *State = nil
			// Take advantage of the following facts:
			// a{5} == aaaaa
			// a{3,5} == aaaa?a?
			// a{5,} == aaaaa+
			// Nov. 3 2024 - I have two choices on how I want to implement numeric
			// specifiers.
			// a. Encode the logic while creating the states. I will have to create a function
			// that creates a deep-copy of a given state / NFA, so that I can concatenate them to
			// each other (concatenating them with the 'concatenate' method - which takes addresses - does
			// not work). Creating this function might be a lot of work.
			// b. Encode the logic while parsing the string (shunting-yard). If I can expand the numeric specifier
			// at this point, I can leave thompson untouched.
			for i := 0; i < c.startReps; i++ { // Case 1
				stateToAdd = concatenate(stateToAdd, cloneState(state))
			}
			if c.endReps == INFINITE_REPS { // Case 3
				s2 := kleene(*state)
				stateToAdd = concatenate(stateToAdd, s2)
			} else { // Case 2
				for i := c.startReps; i < c.endReps; i++ {
					stateToAdd = concatenate(stateToAdd, question(cloneState(state)))
				}
			}
			nfa = append(nfa, stateToAdd)
		}
	}
	if len(nfa) != 1 {
		return Reg{}, fmt.Errorf("Invalid Regex.")
	}

	verifyLastStates(nfa)

	return Reg{nfa[0], numGroups}, nil

}

// Compiles the given regular expression into a Reg type, suitable for use with the
// matching functions. The second return value is non-nil if a compilation error has
// occured. As such, the error value must be checked before using the Reg returned by this function.
// The second parameter is an optional list of flags, passed to the parsing function shuntingYard.
func Compile(re string, flags ...ReFlag) (Reg, error) {
	nodes, err := shuntingYard(re, flags...)
	if err != nil {
		return Reg{}, fmt.Errorf("Error parsing regex: %w", err)
	}
	reg, err := thompson(nodes)
	if err != nil {
		return Reg{}, fmt.Errorf("Error compiling regex: %w", err)
	}
	return reg, nil
}