Question

How can I create a java regular expression for a comma separator list

(3)
(3,6)
(3 , 6 )

I tried, but it does not match anything:

Pattern.compile("\\(\\S[,]+\\)")

and how can I get the value “3” or “3”and “6” in my code from the Matcher class?

Answer 1

Validation regex

You can try this meta-regex approach for clarity:

    String pattern = 
        "< part (?: , part )* >"
            .replace("<", "\\(")
            .replace(">", "\\)")
            .replace(" ", "\\s*")
            .replace("part", "[^\\s*(,)]++");

    System.out.println(pattern);
    /*** this is the pattern
    \(\s*[^\s*(,)]+\s*(?:\s*,\s*[^\s*(,)]+\s*)*\s*\)
    ****/

The part pattern is [^\s(,)]+, i.e. one or more of anything but whitespace, brackets and comma. This construct is called the negated character class. [aeiou] matches any of the 5 vowel letters; [^aeiou] matches everything but (which includes consonants but also numbers, symbols, whitespaces).

The + repetition is also made possessive to ++ for optimization. The (?:...) construct is a non-capturing group, also for optimization.

References

• regular-expressions.info/Character Class, Possessive Quantifier, Non-capturing Group
• java.util.regex.Pattern

---

Testing and splitting

We can then test the pattern as follows:

    String[] tests = {
        "(1,3,6)",
        "(x,y!,a+b=c)",
        "( 1,    3  , 6)",
        "(1,3,6,)",
        "(())",
        "(,)",
        "()",
        "(oh, my, god)",
        "(oh,,my,,god)",
        "([],<>)",
        "(  !!  ,  ??  ,  ++  )",
    };

    for (String test : tests) {
        if (test.matches(pattern)) {
            String[] parts = test
                .replaceAll("^\\(\\s*|\\s*\\)$", "")
                .split("\\s*,\\s*");

            System.out.printf("%s = %s%n",
                test,
                java.util.Arrays.toString(parts)
            );
        } else {
            System.out.println(test + " no match");
        }
    }

This prints:

(1,3,6) = [1, 3, 6]
(x,y!,a+b=c) = [x, y!, a+b=c]
( 1,    3  , 6) = [1, 3, 6]
(1,3,6,) no match
(()) no match
(,) no match
() no match
(oh, my, god) = [oh, my, god]
(oh,,my,,god) no match
([],<>) = [[], <>]
(  !!  ,  ??  ,  ++  ) = [!!, ??, ++]

This uses String.split to get a String[] of all the parts after trimming the brackets out.