MarkdownParse.java

// File reading code from https://howtodoinjava.com/java/io/java-read-file-to-string-examples/
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.ArrayList;

public class MarkdownParse {
    public static int indexOfUnescaped(String str, String search, int startIndex) {
        int currIndex = startIndex - 1;
        int backslashCount;
        do {
            currIndex = str.indexOf(search, currIndex + 1);
            backslashCount = 0;
            int index = currIndex - 1;
            while (index >= 0 && str.charAt(index) == '\\') {
                index--;
                backslashCount++;
            }
        } while (backslashCount % 2 == 1);
        return currIndex;
    }

    public static ArrayList<String> getLinksFromLine(String markdown) {
        ArrayList<String> toReturn = new ArrayList<>();
        // find the next [, then find the ], then find the (, then take up to
        // the next )
        int currentIndex = 0;
        while (currentIndex < markdown.length()) {
            int nextOpenBracket = indexOfUnescaped(markdown, "[", currentIndex);
            if (nextOpenBracket == -1) {
                break;
            }
            int nextCloseBracket = indexOfUnescaped(markdown, "]", nextOpenBracket);
            int openParen = indexOfUnescaped(markdown, "(", nextCloseBracket);
            if (openParen == -1) {
                break;
            }
            int closeParen = indexOfUnescaped(markdown, ")", openParen);
            int nextOpenParen = indexOfUnescaped(markdown, "(", openParen + 1);
            if (nextOpenParen != -1 && closeParen > nextOpenParen) {
                // Invalid link because there's an open paren before the close paren
                int openBracket = indexOfUnescaped(markdown, "[", openParen);
                if (openBracket != -1) {
                    // Maybe there's another link
                    currentIndex = openBracket;
                    continue;
                } else {
                    break;
                }
            }
            if (closeParen == -1) {
                break;
            }
            // Check that this isn't an image link
            if (!(nextOpenBracket > 0 && markdown.charAt(nextOpenBracket - 1) == '!')) {
                String substr = markdown.substring(openParen + 1, closeParen);
                substr = substr.replaceAll("\\\\(.)", "$1");
                toReturn.add(substr);
            }
            currentIndex = closeParen + 1;
        }
        return toReturn;
    }

    public static ArrayList<String> getLinks(String markdown) {
        ArrayList<String> toReturn = new ArrayList<>();
        for (String line : markdown.split("\n")) {
            toReturn.addAll(getLinksFromLine(line));
        }
        return toReturn;
    }

    public static void main(String[] args) throws IOException {
        Path fileName = Path.of(args[0]);
        String contents = Files.readString(fileName);
        ArrayList<String> links = getLinks(contents);
        System.out.println(links);
    }
}