regex/main.go

package main

import (
	"fmt"
	"os"
	"slices"

	"github.com/fatih/color"
)

const CONCAT rune = '~'

func isOperator(c rune) bool {
	if 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)
}

/*
	shuntingYard applies the Shunting-Yard algorithm

to convert the given infix expression to postfix. This makes
it easier to parse the algorithm when doing Thompson.
See: https://blog.cernera.me/converting-regular-expressions-to-postfix-notation-with-the-shunting-yard-algorithm/
*/
func shuntingYard(re string) string {
	re_postfix := make([]rune, 0)
	re_runes := []rune(re)
	/* Add concatenation operators */
	for i := 0; i < len(re_runes); i++ {
		re_postfix = append(re_postfix, re_runes[i])
		if 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_postfix = append(re_postfix, CONCAT)
				}
			}
		}
	}

	// fmt.Println(string(re_postfix))

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

	// Actual algorithm
	for _, c := range re_postfix {
		/* 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 '(', push to opStack
		4. If current character is ')', pop from opStack (and append to outQueue) until '(' is found. Discard parantheses.
		*/
		if isAlphaNum(c) {
			outQueue = append(outQueue, c)
		}
		if isOperator(c) {
			if len(opStack) == 0 {
				opStack = append(opStack, c)
			} else {
				if priority(c) > priority(peek(opStack)) { // 2a
					opStack = append(opStack, c)
				} else {
					for len(opStack) > 0 && priority(c) <= priority(peek(opStack)) { // 2b
						to_append := pop(&opStack)
						outQueue = append(outQueue, to_append)
					}
					opStack = append(opStack, c)
				}
			}
		}
		if c == '(' {
			opStack = append(opStack, c)
		}
		if c == ')' {
			for peek(opStack) != '(' {
				to_append := pop(&opStack)
				outQueue = append(outQueue, to_append)
			}
			_ = pop(&opStack) // Get rid of opening parantheses
		}
	}

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

	return string(outQueue)
}

// Thompson's algorithm. Constructs Finite-State Automaton from given string.
// Returns start state.
func thompson(re string) *State {
	nfa := make([]*State, 0) // Stack of states
	for _, c := range re {
		if isAlphaNum(c) {
			state := State{}
			state.transitions = make(map[int][]*State)
			state.content = int(c)
			state.output = make([]*State, 0)
			state.output = append(state.output, &state)
			state.isEmpty = false
			nfa = append(nfa, &state)
		}
		// Must be an operator if it isn't alphanumeric
		switch c {
		case CONCAT:
			s2 := pop(&nfa)
			s1 := pop(&nfa)
			for i := range s1.output {
				s1.output[i].transitions[s2.content] = append(s1.output[i].transitions[s2.content], s2)
			}
			s1.output = s2.output
			nfa = append(nfa, s1)
		case '*': // Create a 0-state, concat the popped state after it, concat the 0-state after the popped state
			s1 := pop(&nfa)
			stateToAdd := &State{}
			stateToAdd.transitions = make(map[int][]*State)
			stateToAdd.content = EPSILON
			stateToAdd.isEmpty = true
			stateToAdd.isKleene = true
			stateToAdd.output = append(stateToAdd.output, stateToAdd)
			for i := range s1.output {
				s1.output[i].transitions[stateToAdd.content] = append(s1.output[i].transitions[stateToAdd.content], stateToAdd)
			}
			stateToAdd.transitions[s1.content] = append(stateToAdd.transitions[s1.content], s1)
			nfa = append(nfa, stateToAdd)
		case '|':
			s1 := pop(&nfa)
			s2 := pop(&nfa)
			s3 := State{}
			s3.transitions = make(map[int][]*State)
			s3.output = append(s3.output, s1.output...)
			s3.output = append(s3.output, s2.output...)
			s3.transitions[s1.content] = append(s3.transitions[s1.content], s1)
			s3.transitions[s2.content] = append(s3.transitions[s2.content], s2)
			s3.content = EPSILON
			s3.isEmpty = true

			nfa = append(nfa, &s3)
		}
	}
	if len(nfa) != 1 {
		panic("ERROR: Invalid Regex.")
	}

	verifyLastStates(nfa)

	return nfa[0]

}

func main() {
	// Process:
	// 1. Convert regex into postfix notation (Shunting-Yard algorithm)
	// 		a. Add explicit concatenation operators to facilitate this
	// 2. Build NFA from postfix representation (Thompson's algorithm)
	// 3. Run the string against the NFA
	if len(os.Args) < 3 {
		fmt.Println("ERROR: Missing cmdline args")
		os.Exit(22)
	}
	var re string
	re = os.Args[1]
	re_postfix := shuntingYard(re)
	// fmt.Println(re_postfix)
	startState := thompson(re_postfix)
	start, end, matched := match(startState, os.Args[2])
	if matched {
		for i, c := range os.Args[2] {
			if i >= start && i < end {
				color.New(color.FgRed).Printf("%c", c)
			} else {
				fmt.Printf("%c", c)
			}
		}
		fmt.Printf("\n")
	} else {
		fmt.Println(os.Args[2])
	}
}