Matching Patterns not Preceded by Certain Characters with Regular Expressions

In Java, regular expressions offer powerful pattern matching capabilities. One specific task is to match a pattern only when it is not preceded by specified characters.

To achieve this, negative lookbehinds can be employed. Negative lookbehinds use the syntax (?

Example:

Consider the string:

String s = "foobar barbar beachbar crowbar bar ";

To match "bar" only when it is not preceded by "foo," use the following regular expression:

\w*(?<!foo)bar

Here's how it works:

• \w*: Captures any number of word characters (alphabets, numbers, or underscores) before "bar."
• (? Negative lookbehind that ensures the characters "foo" are not present immediately before "bar."

Output:

barbar
beachbar
crowbar
bar

Additional Note:

To capture characters before "bar" (e.g., "beach"), add \w* before capturing "bar":

\w*(?<!foo)\w*bar