Extract a sub string from a string using regular expression

Question

Given the following Java codes:

String test = "'abc,ab,123',,,'123,aa,abc',,,";
Pattern p = Pattern.compile("('\\w\\S+\')");
Matcher m = p.matcher(test);
boolean s = m.matches();
System.out.println(s);

I want to extract all the content in '', for example I want 'abc,ab,123' and '123,aa,abc'. Why the outout is:

false

My regular expression is like: "find a ',followed by a number or a letter, followed by several non-space characters,followed by another '". It should have a match, what's wrong?

matches method tries to match the whole string. If you want to extract some text, you could use find function. — Avinash Raj
– Avinash Raj, Commented Nov 7, 2014 at 17:21

Pshemo · Accepted Answer · 2014-11-07 18:01:03Z

3

Matcher.matches will try to check if regex can match entire string (see the documentation here). Since your regex expects ' at the end, but your string last character is , matches returns false.
If you want to print one or more part of string which matches your regex you need to use Matcher.find method first to let regex engine localize this matching substring. To get next matching substring you can call find again from the same Matcher. To get all matching substrings call find until it returns false as response.

Try:

String test = "'abc,ab,123',,,'123,aa,abc',,,";
Pattern p = Pattern.compile("'[^']*'");//extract non overlapping string between single quotes
Matcher m = p.matcher(test);
while (m.find()) { //does the pattern exists in input (sub)string
  System.out.println(m.group());//print string that matches pattern
}

edited Nov 7, 2014 at 18:01

Pshemo

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answered Nov 7, 2014 at 17:27

SMA

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5 Comments

Maarten Bodewes Over a year ago

Good enough if full string verification is not required and single quotes are always present, so +1

Pshemo Over a year ago

Good job. So you explained a problem. Now to improve your answer even farther you can also describe solution you used. Remember that good answer contains (1) explanation of problem, (2) solution, (3) explanation of solution (even short one will do, remember that you are not posting answer only for OP but also for future readers).

Pshemo Over a year ago

@Liquid If you used it with matches then it fails because last character in string is not ' but ,. If you used it with find then it should match 'abc,ab,123',,,'123,aa,abc' part because .* represents zero-or-more (*) of any-character (.) and because * is greedy. You could try to make * reluctant by adding ? after it, so try with "'\\w.*?'" (BTW you don't need to add \ before ').

Pshemo Over a year ago

@almasshaikh I added some explanation to your answer. Hope you don't mind.

SMA Over a year ago

Thanks @Pshemo. It would be helpful for other's who might face same issue and would find it useful.

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