Simple positive Lookbehind

This is an example of a simple positive look behind. Positive look behind methods assert the existence of a pattern to the left of the position of a String. You can form positive look behinds by opening a noncapturing group with (?<=. Positive looking behind in a String implies that you should:

  • Compile a given String regular expression to a Pattern, using compile(string regex) API method of Pattern. The given regex in the example is a noncapturing group with (?<= a followed by a non white space character.
  • Use matcher(CharSequence input) API method of Pattern to create a Matcher that will match the given String input against this pattern.
  • While the matcher finds the next subsequence of the input sequence that matches the pattern, with find() API method of Matcher get the input subsequence matched, with group() API method of Matcher and print it.

Let’s take a look at the code snippet that follows:

package com.javacodegeeks.snippets.core;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class LookBehind {

    public static void main(String args[]) throws Exception {


  String reg = "(?<=http://)\S+";

  Pattern p = Pattern.compile(reg);

  String str = "";

  Matcher m = p.matcher(str);

  while (m.find()) {

String output = ":" + + ":";




This was an example of a simple positive look behind assertion in Java.

Byron Kiourtzoglou

Byron is a master software engineer working in the IT and Telecom domains. He is an applications developer in a wide variety of applications/services. He is currently acting as the team leader and technical architect for a proprietary service creation and integration platform for both the IT and Telecom industries in addition to a in-house big data real-time analytics solution. He is always fascinated by SOA, middleware services and mobile development. Byron is co-founder and Executive Editor at Java Code Geeks.
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