对Java字符串形成正则表达式

Question

I have a String, where only numbers and none, one or more percentages are allowed

so my regex would be: [\\d+%] , you can test it here

for java i have to transform it,

public static final String regex = "[\\d+\\%]";

and to test it i use this function

public static final String regex = "[\\d+\\%]";

public boolean validate(String myString){

    Pattern pattern = Pattern.compile(regex);
    Matcher matcher = pattern.matcher(myString);

    if (!matcher.matches()) {
        return false;
    }else{
        return true;
    }
}

The regular expression is not working, also if i use

public static final String regex = "[\\d+%]";

Is there any good online tool for escaping a long regular expression for java?

A more advanced question:
the % should be only allowed if a minimum of one digit is in the String, only a % shouldn't be allowed! And: numbers without a % are only allowed if the number of digits is exactly 8, not less (means: 1234567 is bad, but 12345678 is good)

Testcases:

• Bad: % , (empty string), 23b , -1 , 7.5 , %5a , 1 , 1234567
• Good: 12345678 , 23% , 1%53%53 , %7

Answer 1

I have a String, where only numbers and none, one or more percentages are allowed so my regex would be: [\\d+%]

Actually, that matches ONE character which may be a digit, a + or a % .

To match what you have described in words, you need something like this:

  [\d%]*\d[\d%]*

which matches a string containing at least one digit with optional percent signs. Note that the % character is not a meta-character and hence doesn't need to be escaped in the regex. It will match all of the following:

  0
  00
  %0
  0%0
  00%
  0%0%0
  0%%0

and so on, but not just % or any string that contains characters other than digits or % characterss.

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Is there any good online tool for escaping a long regular expression for java?

I'm not aware of one. But escaping wasn't the reason your regex wasn't working.

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A more advanced question: the % should be only allowed if a minimum of one digit is in the String, only a % shouldn't be allowed!

I think my regex above does that. And for the record, here is what it looks like as a Java String literal:

"[\\d%]*\\d[\\d%]*"

Unless you have TAB, NL, CR, etc characters in the regex , it is sufficient to just replace each individual \\ with \\\\ .