如何确定字符串的第一个字符是否为数字？

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最佳答案

Character.isDigit(string.charAt(0))

Note that this will allow any Unicode digit, not just 0-9. You might prefer:

char c = string.charAt(0);
isDigit = (c >= '0' && c <= '9');

Or the slower regex solutions:

s.substring(0, 1).matches("\\d")
// or the equivalent
s.substring(0, 1).matches("[0-9]")

However, with any of these methods, you must first be sure that the string isn't empty. If it is, charAt(0) and substring(0, 1) will throw a StringIndexOutOfBoundsException. startsWith does not have this problem.

To make the entire condition one line and avoid length checks, you can alter the regexes to the following:

s.matches("\\d.*")
// or the equivalent
s.matches("[0-9].*")

If the condition does not appear in a tight loop in your program, the small performance hit for using regular expressions is not likely to be noticeable.

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regular expression starts with number->'^[0-9]'
Pattern pattern = Pattern.compile('^[0-9]');
Matcher matcher = pattern.matcher(String);


if(matcher.find()){


System.out.println("true");
}

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Regular expressions are very strong but expensive tool. It is valid to use them for checking if the first character is a digit but it is not so elegant :) I prefer this way:

public boolean isLeadingDigit(final String value){
final char c = value.charAt(0);
return (c >= '0' && c <= '9');
}

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I just came across this question and thought on contributing with a solution that does not use regex.

In my case I use a helper method:

public boolean notNumber(String input){
boolean notNumber = false;
try {
// must not start with a number
@SuppressWarnings("unused")
double checker = Double.valueOf(input.substring(0,1));
}
catch (Exception e) {
notNumber = true;
}
return notNumber;
}

Probably an overkill, but I try to avoid regex whenever I can.

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IN KOTLIN :

Suppose that you have a String like this :

private val phoneNumber="9121111111"

At first you should get the first one :

val firstChar=phoneNumber.slice(0..0)

At second you can check the first char that return a Boolean :

firstChar.isInt() // or isFloat()

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To verify only first letter is number or character -- For number Character.isDigit(str.charAt(0)) --return true

For character Character.isLetter(str.charAt(0)) --return true