How Can I Use Regex in Java to Match Patterns Only When Not Preceded by Specific Characters?

Matching Patterns not Preceded by Specific Characters using Regex

In Java, regular expressions provide a powerful way to match patterns in strings. Sometimes, you may need to match a pattern only if it is not preceded by certain characters. By leveraging negative lookbehind, regex allows you to achieve this.

Problem:

Consider the following string:

String s = "foobar barbar beachbar crowbar bar ";

We wish to match all occurrences of "bar" that are not immediately preceded by "foo." The desired output would be:

barbar
beachbar
crowbar
bar

Solution:

To accomplish this task, we can use the following regex pattern:

\w*(?<!foo)bar

Breaking down the pattern:

• w* matches any number of word characters (letters, numbers, or underscores) before the potential match. This portion captures characters like "beach" in "beachbar."
• (?

By using this pattern, we can successfully match "bar" occurrences that are not preceded by "foo." The result will mirror the desired output.

Additional Notes:

• For more information on regex negative lookbehind, refer to "Regular Expressions - Lookaround."
• The w* portion was added later to capture the characters before the pattern (e.g., "beach" in "beachbar").

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