Java regex can't work,but javascript can work

Question

My java regex is

Pattern compile = Pattern.compile("^((?:[^\\n]+\\n?)+)\\n*");
String a = "# clj-markdown\n\nA Clojure library designed to ... well, that part is up to you.\n\n## Usage\nFIX==ME==\n\n## License\n\nCopyright © 2015 FIXME\n\nDistributed under the Eclipse Public License either version 1.0 or (at\nyour option) any later version.\n";
System.out.print(compile.matcher(a));
System.out.print(compile.matcher(a).matches());

this output false,but I test in javascript can pass.

/^((?:[^\n]+\n?)+)\n*/.text(b)

how to resolve this problem?

may be this is java regex difference of javascript regex.

@Jongware it won't help since there isn't a dot in the op's regex. — Avinash Raj
– Avinash Raj, Commented Apr 18, 2015 at 12:04

Avinash Raj · Accepted Answer · 2015-04-18 12:02:42Z

1

Turn the in-between \n? to \n*, since \n? matches an optional newline character where \n* matches zero or more newline characters.

String a = "# clj-markdown\n\nA Clojure";
System.out.print(a.matches("^((?:[^\\n]+\\n*)+)\\n*"));

The part where your regex ^((?:[^\\n]+\\n?)+)\\n* fails is markdown\n\nA substring. ie, because of \n\n two newline characters.

(?:[^\\n]+\\n?) matches upto the markdown\n, and the next character in the input string is a newline character , when you make this regex to repeat one or more times (?:[^\\n]+\\n?)+ it would expect any character but not of \n immediately after matching the optional newline character. But there is a newline character following , so this makes the regex to fail.

matches method in java returns true only if the given regex matches the whole input string.

But test function in javascript will return true if the regex given matches from the start (partial or fullmatch but it must be from the start). It won't except full string match.

Example:

> /^[^\n]+\n/.test('foo\n\nbarbuz')
true
> /^[^\n]+\n/.test('\n')
false

edited Apr 18, 2015 at 12:02

answered Apr 18, 2015 at 11:44

Avinash Raj

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1 Comment

SamPeng Over a year ago

Thanks,good job.but,you know why javascript can pass this regex.

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