我如何编写两个函数,如果它以指定的字符/字符串开头或以指定的字符串结尾,那么它们将接受字符串并返回?

例如:

$str = '|apples}';

echo startsWith($str, '|'); //Returns true
echo endsWith($str, '}'); //Returns true

当前回答

这可能奏效

function startsWith($haystack, $needle) {
     return substr($haystack, 0, strlen($needle)) == $needle;
}

资料来源:https://stackoverflow.com/a/4419658

其他回答

我会这样做

     function startWith($haystack,$needle){
              if(substr($haystack,0, strlen($needle))===$needle)
              return true;
        }

  function endWith($haystack,$needle){
              if(substr($haystack, -strlen($needle))===$needle)
              return true;
        }

mpen给出的答案非常彻底,但不幸的是,提供的基准有一个非常重要且有害的监督。

因为针和草堆中的每个字节都是完全随机的,所以针-草堆对在第一个字节上不同的概率为99.609375%,这意味着平均而言,100000对中的大约99609对在第一字节上不同。换言之,基准测试严重偏向于显式检查第一个字节的startswith实现,正如strncmp_startswith2所做的那样。

如果测试生成循环按如下方式实现:

echo 'generating tests';
for($i = 0; $i < 100000; ++$i) {
    if($i % 2500 === 0) echo '.';

    $haystack_length = random_int(1, 7000);
    $haystack = random_bytes($haystack_length);

    $needle_length = random_int(1, 3000);
    $overlap_length = min(random_int(0, $needle_length), $haystack_length);
    $needle = ($needle_length > $overlap_length) ?
        substr($haystack, 0, $overlap_length) . random_bytes($needle_length - $overlap_length) :
        substr($haystack, 0, $needle_length);

    $test_cases[] = [$haystack, $needle];
}
echo " done!<br />";

基准测试结果讲述了一个略有不同的故事:

strncmp_startswith: 223.0 ms
substr_startswith: 228.0 ms
substr_compare_startswith: 238.0 ms
strncmp_startswith2: 253.0 ms
strpos_startswith: 349.0 ms
preg_match_startswith: 20,828.7 ms

当然,这个基准可能仍然不是完全无偏的,但它也测试了当给定部分匹配的针时算法的效率。

简而言之:

function startsWith($str, $needle){
   return substr($str, 0, strlen($needle)) === $needle;
}

function endsWith($str, $needle){
   $length = strlen($needle);
   return !$length || substr($str, - $length) === $needle;
}

2016年8月23日更新

功能

function substr_startswith($haystack, $needle) {
    return substr($haystack, 0, strlen($needle)) === $needle;
}

function preg_match_startswith($haystack, $needle) {
    return preg_match('~' . preg_quote($needle, '~') . '~A', $haystack) > 0;
}

function substr_compare_startswith($haystack, $needle) {
    return substr_compare($haystack, $needle, 0, strlen($needle)) === 0;
}

function strpos_startswith($haystack, $needle) {
    return strpos($haystack, $needle) === 0;
}

function strncmp_startswith($haystack, $needle) {
    return strncmp($haystack, $needle, strlen($needle)) === 0;
}

function strncmp_startswith2($haystack, $needle) {
    return $haystack[0] === $needle[0]
        ? strncmp($haystack, $needle, strlen($needle)) === 0
        : false;
}

测验

echo 'generating tests';
for($i = 0; $i < 100000; ++$i) {
    if($i % 2500 === 0) echo '.';
    $test_cases[] = [
        random_bytes(random_int(1, 7000)),
        random_bytes(random_int(1, 3000)),
    ];
}
echo "done!\n";


$functions = ['substr_startswith', 'preg_match_startswith', 'substr_compare_startswith', 'strpos_startswith', 'strncmp_startswith', 'strncmp_startswith2'];
$results = [];

foreach($functions as $func) {
    $start = microtime(true);
    foreach($test_cases as $tc) {
        $func(...$tc);
    }
    $results[$func] = (microtime(true) - $start) * 1000;
}

asort($results);

foreach($results as $func => $time) {
    echo "$func: " . number_format($time, 1) . " ms\n";
}

结果(PHP 7.0.9)

(从最快到最慢排序)

strncmp_startswith2: 40.2 ms
strncmp_startswith: 42.9 ms
substr_compare_startswith: 44.5 ms
substr_startswith: 48.4 ms
strpos_startswith: 138.7 ms
preg_match_startswith: 13,152.4 ms

结果(PHP 5.3.29)

(从最快到最慢排序)

strncmp_startswith2: 477.9 ms
strpos_startswith: 522.1 ms
strncmp_startswith: 617.1 ms
substr_compare_startswith: 706.7 ms
substr_startswith: 756.8 ms
preg_match_startswith: 10,200.0 ms

启动基准.php

您可以为此使用fnmatch函数。

// Starts with.
fnmatch('prefix*', $haystack);
// Ends with.
fnmatch('*suffix', $haystack);