Including comments in Java regular expressions

Question

I have some complex regular expressions which I need to comment for readability and maintenance. The Java spec is rather terse and I struggled for a long time getting this working. I finally caught my bug and will post it as an answer but I'd be grateful for any other advice on maintaining regexes

As an example I want to comment the subcomponents (of patternS) in a simple name parser:

    String testTarget = "Waldorf T. Flywheel";
    String patternS = "([A-Za-z]+)\\s+([A-Z]\\.)?\\s+([A-Za-z]+)";
    Pattern pattern = Pattern.compile(patternS, Pattern.COMMENTS);
    Assert.assertTrue(pattern.matcher(testTarget).matches());

EDIT: I would be grateful for examples of the (?x) format as well.

EDIT: @geowa4 has a good suggestion which avoids embedded comments. Sinnce java and others have provided for embedded comments what are the cases where they are useful? (I think I have a case but I'd be interested to see others).

EDIT: As noted below @mikej the regex does not support the optional initial well and would be better as:

        String patternS = "([A-Za-z]+)\\s+([A-Z]\\.\\s+)?([A-Za-z]+)";

but that would end up extracting space in the initial

Answer 1

See the post by Martin Fowler on ComposedRegex for some more ideas on improving regexp readability. In summary, he advocates breaking down a complex regexp into smaller parts which can be given meaningful variable names. e.g.

String mandatoryName = "([A-Za-z]+)";
String mandatoryWhiteSpace = "\\s+";
String optionalInitial = "([A-Z]\\.)?";
String pattern = mandatoryName + mandatoryWhiteSpace + optionalInitial +
    mandatoryWhiteSpace + mandatoryName;

Answer 2

I found the following worked:

        String pattern2S = 
            "([A-Za-z]+)      # mandatory firstName\n" +
            "\\s+             # mandatory whitespace\n " +
            "([A-Z]\\.)?      # optional initial\n" +
            "\\s+             # whitespace\n " +
            "([A-Za-z]+)      # mandatory lastName\n"; 

The key thing was to include the newline character \n explicitly in the string

Answer 3

Why don't you just do this:

String pattern2S = 
    "([A-Za-z]+)" + //    mandatory firstName
    "\\s+" +        //    mandatory whitespace
    ...;

CONTINUATION:

If you want to keep the comments with the pattern and you need to read it in from a properties file, use this:

pattern=\
#comment1\\n\
(A-z)\
#comment2\\n\
(0-9)