Regex check if a string contains only digits doesn't work

Question

这是我的代码，我不知道为什么它返回false

    "A123".matches("\\D+");

Answer 1

it must be:

"A123".matches("^\\d+$");

lowercase d stands for a digit ^ for the beginning of the string and ^ for the end of the string.

Answer 2

It is because you are yet another victim among the tens/hundreds of Java devs who are bitten daily by the fact that .matches() is misnamed.

It will try and match the whole input . This is not what you want.

You have to go through a Pattern , a Matcher and .find() instead:

private static final Pattern NONDIGIT = Pattern.compile("\\D");

// Test whether there is any nondigit character in a string:
NONDIGIT.matcher(theString).find();

You should especially do that since anyway .matches() will recompile a Pattern each time; here, you have only one Pattern .

Answer 3

You can try this:

"A123".matches("[0-9]+") 

Also as you have tagged it as Java then there is an alternate way to use NumberUtil.isNumber(String str) from Apache which can check this for you.