Looking ahead and looking behind are called “lookahead” and “lookbehind” assertions, respectively. These assertions allow you to specify conditions that must be satisfied either ahead or behind the current position in the string, without including the matched text in the final result. This can be helpful when you want to find a pattern that is preceded or followed by certain characters or patterns, without including those characters or patterns in the matched result.

To use lookahead or lookbehind assertions in Java regular expressions, you need to use the syntax (?<=...) for lookbehind and (?=...) for lookahead. Inside the parentheses, you can specify the pattern that should be matched ahead or behind, similar to how you would specify a regular expression.

Let’s consider an example where we want to match all occurrences of the word “Java” that are preceded by the word “hello” and followed by the word “world”.

String text = "hello Java world";
String pattern = "(?<=hello )Java(?= world)";

Pattern compiledPattern = Pattern.compile(pattern);
Matcher matcher = compiledPattern.matcher(text);

while (matcher.find()) {
    System.out.println(matcher.group());
}

In this example, we use a lookbehind assertion (?<=hello ) to specify that the word “Java” should be preceded by the word “hello”. We also use a lookahead assertion (?= world) to specify that the word “Java” should be followed by the word “world”. The result will be the matched word “Java” without including the surrounding words “hello” and “world”.

This feature of lookahead and lookbehind assertions can be very useful when dealing with complex string patterns. It allows you to define flexible regex patterns while still being able to precisely control the surrounding context of the matched text.

#Java #RegularExpressions