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
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
PHP 8更新
PHP 8包含了新的str_starts_with和str_ends_with函数,它们最终为这个问题提供了一个高效便捷的解决方案:
$str = "beginningMiddleEnd";
if (str_starts_with($str, "beg")) echo "printed\n";
if (str_starts_with($str, "Beg")) echo "not printed\n";
if (str_ends_with($str, "End")) echo "printed\n";
if (str_ends_with($str, "end")) echo "not printed\n";
该特性的RFC提供了更多信息,同时也讨论了明显(但不那么明显)用户区域实现的优点和问题。
其他回答
专注于startswith,如果您确定字符串不为空,那么在比较之前,在第一个字符、strlen等上添加一个测试会加快速度:
function startswith5b($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。
这里有一个针对PHP4的有效解决方案。如果在PHP5上使用substra_compare而不是strcasecmp(substr(…)),可以获得更快的结果。
function stringBeginsWith($haystack, $beginning, $caseInsensitivity = false)
{
if ($caseInsensitivity)
return strncasecmp($haystack, $beginning, strlen($beginning)) === 0;
else
return strncmp($haystack, $beginning, strlen($beginning)) === 0;
}
function stringEndsWith($haystack, $ending, $caseInsensitivity = false)
{
if ($caseInsensitivity)
return strcasecmp(substr($haystack, strlen($haystack) - strlen($ending)), $haystack) === 0;
else
return strpos($haystack, $ending, strlen($haystack) - strlen($ending)) !== false;
}
Fastest endsWith()解决方案:
# Checks if a string ends in a string
function endsWith($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
为什么不采取以下措施?
//How to check if a string begins with another string
$haystack = "valuehaystack";
$needle = "value";
if (strpos($haystack, $needle) === 0){
echo "Found " . $needle . " at the beginning of " . $haystack . "!";
}
输出:
在valuehaystack的开头找到值!
请记住,如果在干草堆中没有找到针,strpos将返回false,如果且仅当在索引0处找到针时(即开头),strpos将返回0。
以下是结尾:
$haystack = "valuehaystack";
$needle = "haystack";
//If index of the needle plus the length of the needle is the same length as the entire haystack.
if (strpos($haystack, $needle) + strlen($needle) === strlen($haystack)){
echo "Found " . $needle . " at the end of " . $haystack . "!";
}
在这种情况下,不需要函数startsWith()作为
(strpos($stringToSearch, $doesItStartWithThis) === 0)
将准确返回true或false。
奇怪的是,这么简单,所有的野生函数都在这里泛滥。
您可以使用strpos和strrpos
$bStartsWith = strpos($sHaystack, $sNeedle) == 0;
$bEndsWith = strrpos($sHaystack, $sNeedle) == strlen($sHaystack)-strlen($sNeedle);