How to Match Commas Outside Parentheses Using Java Regular Expressions?

Matching Commas Outside Parentheses with Java Regular Expressions

Given a string with commas potentially enclosed within parentheses, the objective is to identify commas that are not contained within any parenthesis grouping. This can be achieved through a customized regular expression.

Java Regex Solution

Assuming the absence of nested parentheses within the input string, the following Java regular expression can be utilized:

Pattern regex = Pattern.compile(
    ",         # Match a comma\n" +
    "(?!       # only if it's not followed by...\n" +
    " [^(]*    #   any number of characters except opening parens\n" +
    " \)      #   followed by a closing parens\n" +
    ")         # End of lookahead", 
    Pattern.COMMENTS);

This regex pattern leverages a negative lookahead assertion to verify that the next consecutive parenthesis (if present) is not a closing parenthesis. Only in that scenario is the comma allowed to be matched.

Usage and Explanation

The pattern is divided into two main components:

1. Matching the Comma: The regular expression begins with a simple match for any standalone comma.
2. Negative Lookahead Assertion: The following portion, enclosed within parentheses and preceded by (?!, asserts that the character sequence that follows the comma should not match the specified pattern. In this case, it ensures that not all characters up to and including the next closing parenthesis (represented by [^)]* \)) are present.

By excluding commas enclosed within parentheses, this regex effectively captures only the commas that exist outside any parentheses grouping.

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