How check that string is NOT blank by java regular expression?

Question

There is regular expression for finding blank string and I want only negation. I also see this question but it does not work for java (see examples). Solution also not work for me (see 3-rd line in example).

For example

Pattern.compile("/^$|\\s+/").matcher(" ").matches() - false
Pattern.compile("/^$|\\s+/").matcher(" a").matches()- false
Pattern.compile("^(?=\\s*\\S).*$").matcher("\t\n a").matches() - false

return false in both cases.

P.S. If something is not clear ask me questions.

UPDATED

I want to use this regular expression in @Pattern annotation without creating custom annotation and programmatic validator for it. That's why I want a "plain" regexp solution without using find function.

Gene · Answer 1 · 2013-08-27T05:50:24.183

5

It's not clear what you mean by negation.

If you mean "a string that contains at least one non-blank character," then you can use this:

Pattern.compile("\\S").matcher(str).find()

If it's really necessary to use matches, then you can do it with this.

Pattern.compile("\\A\\s*\\S.*\\Z").matcher(str).matches()

This just matches 0 or more spaces followed by a non-space followed by any characters at all up to the end of the string.

If you mean "a string that is all non-blank with at least one such character," then you can use this:

Pattern.compile("\\A\\S+\\Z").matcher(str).matches()

You need to study the Java regex syntax. In Java, regular expressions are compiled from strings, so there's no need for special delimiters like /.../ or %r{...} as you'll see in other languages.

edited Aug 27 '13 at 05:50

answered Aug 21 '13 at 12:07

Gene

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Congratulations to your 10k! – Joachim Sauer Aug 21 '13 at 12:11
Pattern.compile("\\S").matcher("abc").matches() - false, but must be true. Your solution is only for 1 character in the string. – Cherry Aug 22 '13 at 02:24
2

@Cherry Use `find` instead of `match`. I edited the answer to make this explicit. – Gene Aug 22 '13 at 02:34
1

@Cherry Then what do you mean by negation? The opposite of a string of all spaces is a string that is not all spaces, which is a string that contains at least one non-space. That's what I gave you the regex for... – Gene Aug 22 '13 at 02:42
The main reason why I added this question is possibility to use [@Pattern](http://docs.oracle.com/javaee/6/api/javax/validation/constraints/Pattern.html) to ckeck that input string is not blank. So I do not want to use find() function. I can create my own annotation and write a validator, but I asked to check is there solution in "plain" rexexp or not. – Cherry Aug 27 '13 at 04:32
@Cherry Of course it doesn't. As my answer says, this finds all strings that consist of all _non-blank_ characters, where there is at least one of these. – Gene Aug 27 '13 at 05:33
@Cherry Ok I added a regex that uses `matches` to match a line with at least one non-blank in it. – Gene Aug 27 '13 at 05:51

score 1 · Answer 2 · answered Aug 21 '13 at 12:20

1

How about this:

if(!string.trim().isEmpty()) {
    // do something
}

answered Aug 21 '13 at 12:20

Sethiel

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If string is null, throws [`java.lang.NullPointerException`](http://docs.oracle.com/javase/7/docs/api/java/lang/NullPointerException.html). – Paul Vargas Aug 21 '13 at 14:12
Updated question description. – Cherry Aug 27 '13 at 04:39

Ankur Lathi · Answer 3 · 2013-08-21T12:38:28.717

0

Use regex \s : A whitespace character: \t\n\x0B\f\r.

Pattern.compile("\\s")

edited Aug 21 '13 at 12:38

answered Aug 21 '13 at 12:13

Ankur Lathi

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Works only for string with 1 character. – Cherry Aug 27 '13 at 04:39

How check that string is NOT blank by java regular expression?

3 Answers3