如何确定字符串的第一个字符是否为数字？ [英] How do I find out if first character of a string is a number?

问题描述

在Java中有没有办法找出字符串的第一个字符是否为数字？

In Java is there a way to find out if first character of a string is a number?

一种方法是

string.startsWith("1")

并一直做到9点，但这似乎效率很低。

and do the above all the way till 9, but that seems very inefficient.

推荐答案

Character.isDigit(string.charAt(0))

请注意这将允许任何 Unicode数字，而不仅仅是0-9。您可能更喜欢：

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');

或更慢的正则表达式解决方案：

Or the slower regex solutions:

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

但是，使用上述任何一种方法，您必须先确保字符串不为空。如果是， charAt（0）和 substring（0,1）将抛出的StringIndexOutOfBoundsException 。 startsWith 没有此问题。

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.

要使整个条件一行并避免长度检查，您可以更改正则表达式如下：

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].*")

如果条件没有出现在程序的紧密循环中，那么使用正则表达式的性能影响不大可能会很明显。

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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