Regex for matching something if it is not preceded by something else

java

regex-engineering

lookbehind

performance

byAnton Shumikhin·Oct 18, 2024

To enable a regex to match a string "target" that is not preceded by "notThis", you need to employ the negative lookbehind: "(?<!notThis)target".

String regex = "(?<!notThis)target";  // No "target" after "notThis", please.

To implement this using Java, you would follow the procedure below:

String sample = "I want this target but notThis target"; // choose your "target" wisely.
Pattern pattern = Pattern.compile("(?<!notThis)target"); // I've got my eyes on you, "notThis"!
Matcher matcher = pattern.matcher(sample);

while (matcher.find()) {
    System.out.println(matcher.group());  // Only prints "target" when "notThis" isn't lurking around.
}

Bear in mind, Java's lookbehind needs a fixed or max length to work - it's just not the experimental type!

Lookbehind in detail

Dealing with complicated strings

For more complex strings, you can even use alternations inside the negative lookbehinds. So, don't shy away from complicating things! Increase your regex skills and become the master of lookbehinds.

Validate before you regret

Make it a practice to test your regex patterns with tools like regex101.com. It gives you real-time feedback - consider it your regex lifeguard, saving you from drowning in a sea of failed matches.

When regex says no, you say yes!

Java regex might throw you a curveball, and limitations with lookbehinds may keep you back - but only if you let them. If your pattern has a variable length, try breaking down your problem or use code to filter out unwanted matches.

Regex and code, best buds

When regex can’t do it all, Java's string manipulation methods got your back! Combine them with regex for maximum control and see your matches shine!

Lookbehind tricks and tips

Lean lookbehinds

Java's regex engine demands your lookbehind to have a definite end. So, keep it lean or face the regex wrath!

A little bit of this, a little bit of that

Conditional checks with lookarounds can help you achieve sophisticated matching. Just remember not to capture your unwanted text along with the good stuff.

Fast and furious matching

Regex lookarounds can be heavyweight champions in terms of computational expense, so strike a balance between complexity and performance. Remember, optimization is key.

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