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

专注于startswith,如果您确定字符串不为空,那么在比较之前,在第一个字符、strlen等上添加一个测试会加快速度:

function startswith5b($haystack, $needle) {
    return ($haystack{0}==$needle{0})?strncmp($haystack, $needle, strlen($needle)) === 0:FALSE;
}

不知怎么的,速度快了(20%-30%)。添加另一个字符测试,如$haystack{1}==$needle{1},似乎并不能加快速度,甚至可能会减慢速度。

==似乎比==条件运算符(a)?b: c似乎比如果(a)b更快;否则c;


对于那些问“为什么不使用strpos?”并称其他解决方案为“不必要的工作”的人


strpos速度很快,但它不是这项工作的正确工具。

为了理解,这里有一个小模拟作为示例:

Search a12345678c inside bcdefga12345678xbbbbb.....bbbbba12345678c

电脑在里面做什么?

    With strccmp, etc...

    is a===b? NO
    return false



    With strpos

    is a===b? NO -- iterating in haysack
    is a===c? NO
    is a===d? NO
    ....
    is a===g? NO
    is a===g? NO
    is a===a? YES
    is 1===1? YES -- iterating in needle
    is 2===3? YES
    is 4===4? YES
    ....
    is 8===8? YES
    is c===x? NO: oh God,
    is a===1? NO -- iterating in haysack again
    is a===2? NO
    is a===3? NO
    is a===4? NO
    ....
    is a===x? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    ...
    ... may many times...
    ...
    is a===b? NO
    is a===a? YES -- iterating in needle again
    is 1===1? YES
    is 2===3? YES
    is 4===4? YES
    is 8===8? YES
    is c===c? YES YES YES I have found the same string! yay!
    was it at position 0? NOPE
    What you mean NO? So the string I found is useless? YEs.
    Damn.
    return false

假设strlen不迭代整个字符串(但即使在这种情况下),这一点都不方便。

其他回答

这个问题已经有很多答案,但在某些情况下,你可以选择比所有答案都简单的答案。如果要查找的字符串是已知的(硬编码的),则可以使用正则表达式而不使用任何引号等。

检查字符串是否以“ABC”开头:

preg_match('/^ABC/', $myString); // "^" here means beginning of string

以“ABC”结尾:

preg_match('/ABC$/', $myString); // "$" here means end of string

在我的简单示例中,我想检查字符串是否以斜杠结尾:

preg_match('#/$#', $myPath);   // Use "#" as delimiter instead of escaping slash

优点:由于它非常简短和简单,您不必像上面所示那样定义函数(例如endsWith())。

但是,这并不是每一种情况的解决方案,只是一种非常具体的解决方案。

简短易懂的一行,没有正则表达式。

startsWith()是直接向前的。

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

endsWith()使用稍微花哨且缓慢的strrev():

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

Fastest endsWith()解决方案:

# Checks if a string ends in a string
function endsWith($haystack, $needle) {
    return substr($haystack,-strlen($needle))===$needle;
}

基准:

# This answer
function endsWith($haystack, $needle) {
    return substr($haystack,-strlen($needle))===$needle;
}

# Accepted answer
function endsWith2($haystack, $needle) {
    $length = strlen($needle);

    return $length === 0 ||
    (substr($haystack, -$length) === $needle);
}

# Second most-voted answer
function endsWith3($haystack, $needle) {
    // search forward starting from end minus needle length characters
    if ($needle === '') {
        return true;
    }
    $diff = \strlen($haystack) - \strlen($needle);
    return $diff >= 0 && strpos($haystack, $needle, $diff) !== false;
}

# Regex answer
function endsWith4($haystack, $needle) {
    return preg_match('/' . preg_quote($needle, '/') . '$/', $haystack);
}

function timedebug() {
    $test = 10000000;

    $time1 = microtime(true);
    for ($i=0; $i < $test; $i++) {
        $tmp = endsWith('TestShortcode', 'Shortcode');
    }
    $time2 = microtime(true);
    $result1 = $time2 - $time1;

    for ($i=0; $i < $test; $i++) {
        $tmp = endsWith2('TestShortcode', 'Shortcode');
    }
    $time3 = microtime(true);
    $result2 = $time3 - $time2;

    for ($i=0; $i < $test; $i++) {
        $tmp = endsWith3('TestShortcode', 'Shortcode');
    }
    $time4 = microtime(true);
    $result3 = $time4 - $time3;

    for ($i=0; $i < $test; $i++) {
        $tmp = endsWith4('TestShortcode', 'Shortcode');
    }
    $time5 = microtime(true);
    $result4 = $time5 - $time4;

    echo $test.'x endsWith: '.$result1.' seconds # This answer<br>';
    echo $test.'x endsWith2: '.$result4.' seconds # Accepted answer<br>';
    echo $test.'x endsWith3: '.$result2.' seconds # Second most voted answer<br>';
    echo $test.'x endsWith4: '.$result3.' seconds # Regex answer<br>';
    exit;
}
timedebug();

基准结果:

10000000x endsWith: 1.5760900974274 seconds # This answer
10000000x endsWith2: 3.7102129459381 seconds # Accepted answer
10000000x endsWith3: 1.8731069564819 seconds # Second most voted answer
10000000x endsWith4: 2.1521229743958 seconds # Regex answer
function startsWith($haystack, $needle, $case = true) {
    if ($case) {
        return (strcmp(substr($haystack, 0, strlen($needle)), $needle) === 0);
    }
    return (strcasecmp(substr($haystack, 0, strlen($needle)), $needle) === 0);
}

function endsWith($haystack, $needle, $case = true) {
    if ($case) {
        return (strcmp(substr($haystack, strlen($haystack) - strlen($needle)), $needle) === 0);
    }
    return (strcasecmp(substr($haystack, strlen($haystack) - strlen($needle)), $needle) === 0);
}

贷方:

检查字符串是否以另一个字符串结尾

检查字符串是否以另一个字符串开头

您可以使用strpos和strrpos

$bStartsWith = strpos($sHaystack, $sNeedle) == 0;
$bEndsWith = strrpos($sHaystack, $sNeedle) == strlen($sHaystack)-strlen($sNeedle);