Parse Java code between braces using Regex and groups

Question

I am creating some java code that takes correctly written .java files as input, and i want to extract the text between braces using a regular expression. I want to use the Pattern and Matcher classes, and not for loops.

I believe its best to create a regex that groups the text in the whole class, and later another regex that will be aplied to the previous output and groups the text in methods.

I got close to getting the class text using the following regex on online regex testers:

\w\sclass.*\{((.*\s*)*)\}

but i'm pretty sure i am doing it wrong by using two groups instead of just one. Furthermore when i use this expression in Java i am actually getting nothing.

Here is an example file that i am using for debugging

package foo.bar;

import java.io.File;

public class Handy {
    {
    // static block, dont care!
    }

    /**
     * Check if a string is Null and Empty
     * @param str
     * @return
     */
    public static boolean isNullOrEmpty(String str) {
        Boolean result = (str == null || str.isEmpty());
        return result;
    }

    /**
     * Mimics the String.format method with a smaller name
     * @param format
     * @param args
     * @return
     */
    public static String f(String format, Object... args)
    {
        return String.format(format, args);
    }
}

With the example code above, i expect to get:

entire class text

{
// static block, dont care!
}

/**
 * Check if a string is Null and Empty
 * @param str
 * @return
 */
public static boolean isNullOrEmpty(String str) {
    Boolean result = (str == null || str.isEmpty());
    return result;
}

/**
 * Mimics the String.format method with a smaller name
 * @param format
 * @param args
 * @return
 */
public static String f(String format, Object... args)
{
    return String.format(format, args);
}

individual method text

Boolean result = (str == null || str.isEmpty());
return result;

return String.format(format, args);

I know how to use the Pattern and Matcher classes already, i just need the right regexes...

You sort of forgot to tell us exactly what you want to match here, but that doesn't really matter, because regex is not a suitable tool for parsing nested source code, which is what Java is. — Tim Biegeleisen
– Tim Biegeleisen, Commented May 6, 2019 at 14:00
Your match is in the first capturing group regex101.com/r/M47iI9/2 — The fourth bird
– The fourth bird, Commented May 6, 2019 at 14:01
Regex is not for code parsing. That should be done with lexers/parsers. — duffymo
– duffymo, Commented May 6, 2019 at 14:20
Nope. You shouldn't avoid. There's a quip that says "You've got a problem. You think regex is a good solution. Now you have two problems." True for your case. — duffymo
– duffymo, Commented May 6, 2019 at 14:27
@Aaron just "parsing" character by character could get complex too though: you'd need to distinguish between initializer blocks, classes (outer and inner), braces in comments such as JavaDoc's {@code}, braces in string literals (not that uncommon), methods, blocks inside methods etc. — Thomas
– Thomas, Commented May 6, 2019 at 14:45

Luis Ferreira · Accepted Answer · 2019-05-09 11:54:19Z

After some confusion in the comments section, i would like to share my solution for what i asked, even if it was not very clear.

This is not thoroughly tested code, but it works for my purpose. Some adjustments or improvements are very likely possible. I took some inspiration from the comments i read in this post, and others like this.

I feed each of the following methods the entire plain text found in a .java file, and from there i use Pattern and Matcher to extract what i want.

private static String patternMatcher(String content, String patternText, int groupIndex) {
    Pattern pattern = Pattern.compile(patternText);
    Matcher matcher = pattern.matcher(content);

    if (matcher.find()) {
        return matcher.group(groupIndex);
    } else {
        return "";
    }
}

public static String getPackageName(String content) {
    return patternMatcher(content, ".*package\\s+(.*)\\s*\\;", 1);
}

public static String getClassName(String content) {
    return patternMatcher(content, ".*class\\s+(\\w+)[\\w\\s]+\\{", 1);
}

public static String getClassCode(String content) {
    return patternMatcher(content, ".*class.*\\{((.*\\s*)*)\\}", 1);
}

public static String getMethodName(String code) {
    String uncommentedCode = removeComments(code).trim();

    return patternMatcher(uncommentedCode,
            "(public|private|static|protected|abstract|native|synchronized) *([\\w<>.?, \\[\\]]*)\\s+(\\w+)\\s*\\([\\w<>\\[\\]._?, \\n]*\\)\\s*([\\w ,\\n]*)\\s*\\{",
            3);
}

public static String removeComments(String content) {
    return content.replaceAll("\\/\\*[\\s\\S]*?\\*\\/|([^:]|^)\\/\\/.*$", "$1 ").trim();
}

I double checked but i hope i didn't forget any escape character, be carefull with those.

Lots of people recomended that i used an actual code parsing library, like ANTLR, but i assumed it would take much longer for me to learn how to work with it, then it would take to do with with RegEx. Furthermore i wanted to improve my Regex skills, this exercise definitely taught me some things.

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