正则表达式对小字符串的暴力

public static boolean isHexadecimal(String value)
{
    if (value.startsWith("-"))
    {
        value = value.substring(1);
    }

    value = value.toLowerCase();

    if (value.length() <= 2 || !value.startsWith("0x"))
    {
        return false;
    }

    for (int i = 2; i < value.length(); i++)
    {
        char c = value.charat(i);

        if (!(c >= '0' && c <= '9' || c >= 'a' && c <= 'f'))
        {
            return false;
        }
    }

    return true;
}

Regex.match(/0x[0-9a-f]+/,"0x123fa") // returns true if regex matches whole given expression

我写了一个小基准来估计：

> nop方法(了解基线迭代速度);
>原始方法,由OP提供;
> RegExp;
>编译正则表达式;
> @maraca提供的版本(没有toLowerCase和substring);
>“fastIsHex”版本(基于开关),我添加的只是为了好玩.

测试机配置如下：

> JVM：Java(TM)SE运行时环境(版本1.8.0_101-b13)
> cpu：Intel(R)Core(TM)i5-2500 cpu @ 3.30GHz

以下是我为原始测试字符串“0x123fa”和10.000.000次迭代得到的结果：

Method "nop" => #10000000 iterations in 9ms
Method "isHexadecimal (OP)" => #10000000 iterations in 300ms
Method "RegExp" => #10000000 iterations in 4270ms
Method "RegExp (Compiled)" => #10000000 iterations in 1025ms
Method "isHexadecimal (maraca)" => #10000000 iterations in 135ms
Method "fastIsHex" => #10000000 iterations in 107ms

正如您所看到的,OP的原始方法比RegExp方法更快(至少在使用JDK提供的RegExp实现时).

(供你参考)

基准代码：

public static void main(String[] argv) throws Exception {
    //Number of IteraTIONS
    final int IteraTIONS = 10000000;

    //nop
    benchmark(IteraTIONS,"nop",() -> nop(longHexText));

    //isHexadecimal
    benchmark(IteraTIONS,"isHexadecimal (OP)",() -> isHexadecimal(longHexText));

    //Un-compiled regexp
    benchmark(IteraTIONS,"RegExp",() -> longHexText.matches("0x[0-9a-fA-F]+"));

    //Pre-compiled regexp
    final Pattern pattern = Pattern.compile("0x[0-9a-fA-F]+");
    benchmark(IteraTIONS,"RegExp (Compiled)",() -> {
        pattern.matcher(longHexText).matches();
    });

    //isHexadecimal (maraca)
    benchmark(IteraTIONS,"isHexadecimal (maraca)",() -> isHexadecimalMaraca(longHexText));

    //FastIsHex
    benchmark(IteraTIONS,"fastIsHex",() -> fastIsHex(longHexText));
}

public static void benchmark(int iterations,String name,Runnable block) {
    //Start Time
    long stime = System.currentTimeMillis();

    //Benchmark
    for(int i = 0; i < iterations; i++) {
        block.run();
    }

    //Done
    System.out.println(
        String.format("Method \"%s\" => #%d iterations in %dms",name,iterations,(System.currentTimeMillis()-stime))
    );
}

nop方法：

public static boolean nop(String value) { return true; }

fastIsHex方法：

public static boolean fastIsHex(String value) {

    //Value must be at least 4 characters long (0x00)
    if(value.length() < 4) {
        return false;
    }

    //Compute where the data starts
    int start = ((value.charat(0) == '-') ? 1 : 0) + 2;

    //Check prefix
    if(value.charat(start-2) != '0' || value.charat(start-1) != 'x') {
        return false;
    }

    //Verify data
    for(int i = start; i < value.length(); i++) {
        switch(value.charat(i)) {
            case '0':case '1':case '2':case '3':case '4':case '5':case '6':case '7':case '8':case '9':
            case 'a':case 'b':case 'c':case 'd':case 'e':case 'f':
            case 'A':case 'B':case 'C':case 'D':case 'E':case 'F':
                continue;

            default:
                return false;
        }
    }

    return true;
}

所以,答案是否定的,对于短字符串和手头的任务,RegExp并不快.

当谈到更长的弦乐时,平衡是完全不同的,
下面是8192长十六进制字符串的结果,我用以下代码生成：

hexdump -n 8196 -v -e '/1 "%02X"' /dev/urandom

和10.000次迭代：

Method "nop" => #10000 iterations in 2ms
Method "isHexadecimal (OP)" => #10000 iterations in 1512ms
Method "RegExp" => #10000 iterations in 1303ms
Method "RegExp (Compiled)" => #10000 iterations in 1263ms
Method "isHexadecimal (maraca)" => #10000 iterations in 553ms
Method "fastIsHex" => #10000 iterations in 530ms

正如你所看到的,手写方法(由macara和我的fastIsHex编写的方法)仍然击败RegExp,但原始方法没有,
(由于substring()和toLowerCase()).

边注：

这个基准测试非常简单,只测试“最坏情况”场景(即一个完全有效的字符串),实际结果,混合数据长度和非0有效无效比率可能完全不同.

更新：

我还尝试了char []数组版本：

char[] chars = value.tochararray();
 for (idx += 2; idx < chars.length; idx++) { ... }

它甚至比getCharat(i)版本慢一点：

Method "isHexadecimal (maraca) char[] array version" => #10000000 iterations in 194ms
  Method "fastIsHex,char[] array version" => #10000000 iterations in 164ms

我的猜测是由于tochararray中的数组复制.

更新(#2)：

我已经运行了额外的8k / 100.000迭代测试,看看“maraca”和“fastIsHex”方法之间的速度是否存在任何真正的差异,并且还将它们标准化为使用完全相同的前置条件代码：

运行#1

Method "isHexadecimal (maraca) *normalized" => #100000 iterations in 5341ms
Method "fastIsHex" => #100000 iterations in 5313ms

运行#2

Method "isHexadecimal (maraca) *normalized" => #100000 iterations in 5313ms
Method "fastIsHex" => #100000 iterations in 5334ms

即这两种方法之间的速度差异最多是微不足道的,可能是由于测量错误(因为我在我的工作站上运行它而不是专门设置的清洁测试环境).