Using Java to find substring of a bigger string using Regular Expression

java

regex

string-processing

advanced-matching

byNikita Barsukov·Mar 9, 2025

To extract a substring in Java, leverage the Pattern and Matcher classes. Use matcher.find() to locate your regex pattern, and matcher.group() to retrieve it. Here's a concise, effective snippet:

String input = "Java Sage says: The quick brown fox jumps over the lazy dog.";
String regex = "quick (\\S+) fox";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(input);

// Our curious fox found something!
if (matcher.find()) {
    System.out.println(matcher.group(1)); // Outputs "brown", the fox's secret color!
}

This snippet finds the word between "quick" and "fox". Adapt the regex pattern to meet your specific needs and extract substrings with precision.

Greedy vs non-greedy matching

Non-greedy quantifiers like *? help to prevent overmatching. They ensure the match is the shortest possible, part of the advanced matching techniques that keep your code performing at its peak.

Multiple occurrences? No problem! Regular code repetition can wear out your keys. Use a while loop with Matcher.find() to keep things rolling smoothly:

String input = "[Caution] Care with brackets [extract me]";
String regex = "\\[(.*?)\\]";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(input);

List<String> matches = new ArrayList<>();
// Time to play the matching game!
while (matcher.find()) {
    matches.add(matcher.group(1)); // Adds "Caution", "extract me" to the party list
}

matches.forEach(System.out::println); // Spills the party beans!

For special characters like brackets, check out the Pattern API Documentation. When regex talks in special codes, knowing how to escape characters is a lifesaver.

Dealing with edge cases

Strings can sometimes act like chainsaws and throw you off balance with nested or unbalanced brackets. With just regex, you might step into a slippery slope. Solution? Brace yourself with more elaborate regex patterns or string processing techniques.

Advanced regex techniques

Finding treasures within treasures

When dealing with nested structures within strings, you might need a more intricate clause for regex. Consider the following pattern:

String regex = "\\[(?:\\[(.*?)\\]|[^\\[])*?\\]";

Like an explorer finding a cave within a cave on an adventure, this pattern helps to find matches within nested brackets. Not to be overused, though, as your regex can get too lost in the layers!

Keep or kick the brackets

To exclude the enclosing brackets from the match, you might consider this "kick-the-bracket" regex:

String regex = "\\[([^\\]]*)\\]";

In this case, [^\\]]* matches anything but not the closing bracket, effectively stopping just before it. Not a fan of cheesy JPEG endings? No problem!

Encapsulate with style

To keep your code clean and maintainable, condense the logic into a method:

public List<String> extractSubstrings(String input, String regex) {
    Pattern pattern = Pattern.compile(regex);
    Matcher matcher = pattern.matcher(input);
    List<String> matches = new ArrayList<>();
    
    // Get ready for an extraction
    while (matcher.find()) {
        matches.add(matcher.group(1));
    }
    
    // Giving back is a good practice!
    return matches;
}

Looks neat, functions efficiently, and keeps complexity at bay.

Twists, traps and tips

Be wary of balanced brackets and their oddities. Always ready yourself for unexpected inputs and remember to check the return value of matcher.find() to avoid an IllegalStateException.

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