How to Match Commas Outside Parentheses Using Regular Expressions in Java?

Matching Commas Outside Parentheses Using Regular Expressions

Problem:

Consider a string containing multiple commas, such as "12,44,foo,bar,(23,45,200),6." The goal is to create a regular expression in Java that matches all commas except those enclosed within parentheses, i.e., only the commas after 23 and 45 in the given example.

Solution:

To achieve this, we need to use a regular expression that incorporates a negative lookahead assertion. Here's a Java regular expression that satisfies this requirement:

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);

Explanation:

1. Comma Match: The first part of the regex, "commas", simply matches a comma.
2. Negative Lookahead Assertion: The negative lookahead assertion, enclosed within the "(?!)" section, checks whether the following pattern does not match the comma.
3. Nested Parentheses Check: This pattern consists of "¹ )". The "¹" part matches any number of characters except an opening parenthesis "(". The ")" part matches a closing parenthesis ")".
4. Overall Logic: The negative lookahead ensures that the comma is only matched if it is not followed by any characters except opening parentheses, which are then followed by a closing parenthesis. This effectively excludes commas within parentheses.

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