php 如何检查字符串是否以指定的字符串开头？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/2790899/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How to check if a string starts with a specified string?
提问by Andrew
I'm trying to check if a string starts with http. How can I do this check?
我正在尝试检查字符串是否以http. 我怎样才能做这个检查？
$string1 = 'google.com';
$string2 = 'http://www.google.com';
回答by Kendall Hopkins
Use the substrfunction to return a part of a string.
使用substr函数返回字符串的一部分。
substr( $string_n, 0, 4 ) === "http"
If you're trying to make sure it's not another protocol. I'd use http://instead, since https would also match, and other things such as http-protocol.com.
如果您想确保它不是另一种协议。我会http://改用，因为 https 也会匹配，还有其他的东西，比如 http-protocol.com。
substr( $string_n, 0, 7 ) === "http://"
And in general:
一般来说：
substr($string, 0, strlen($query)) === $query
回答by awgy
Use strpos():
使用strpos()：
if (strpos($string2, 'http') === 0) {
// It starts with 'http'
}
Remember the three equals signs (===). It will not work properly if you only use two. This is because strpos()will return falseif the needle cannot be found in the haystack.
记住三个等号 ( ===)。如果您只使用两个，它将无法正常工作。这是因为如果在大海捞针中找不到针，strpos()它将返回false。
回答by Sid
There is also the strncmp()function and strncasecmp()function which is perfect for this situation:
还有一个strncmp()功能和strncasecmp()功能非常适合这种情况：
if (strncmp($string_n, "http", 4) === 0)
In general:
一般来说：
if (strncmp($string_n, $prefix, strlen($prefix)) === 0)
The advantage over the substr()approach is that strncmp()just does what needs to be done, without creating a temporary string.
相对于这种substr()方法的优点是strncmp()只做需要做的事情，而不创建临时字符串。
回答by user276648
You can use a simple regex (updated version from user viriathusas eregiis deprecated)
您可以使用一个简单的regex（更新版本从用户viriathus为eregi已废弃）
if (preg_match('#^http#', $url) === 1) {
// Starts with http (case sensitive).
}
or if you want a case insensitive search
或者如果您想要不区分大小写的搜索
if (preg_match('#^http#i', $url) === 1) {
// Starts with http (case insensitive).
}
Regexes allow to perform more complex tasks
正则表达式允许执行更复杂的任务
if (preg_match('#^https?://#i', $url) === 1) {
// Starts with http:// or https:// (case insensitive).
}
Performance wise, you don't need to create a new string (unlike with substr) nor parse the whole string if it doesn't start with what you want. You will have a performance penalty though the 1st time you use the regex (you need to create/compile it).
在性能方面，您不需要创建新字符串（与 substr 不同），也不需要解析整个字符串，如果它不是以您想要的开头。尽管第一次使用正则表达式（您需要创建/编译它），但您会受到性能损失。
This extension maintains a global per-thread cache of compiled regular expressions (up to 4096). http://www.php.net/manual/en/intro.pcre.php
此扩展维护已编译正则表达式的全局每线程缓存（最多 4096 个）。 http://www.php.net/manual/en/intro.pcre.php
回答by Jake
You can check if your string starts with http or https using the small function below.
您可以使用下面的小函数检查您的字符串是否以 http 或 https 开头。
function has_prefix($string, $prefix) {
return substr($string, 0, strlen($prefix)) == $prefix;
}
$url = 'http://www.google.com';
echo 'the url ' . (has_prefix($url, 'http://') ? 'does' : 'does not') . ' start with http://';
echo 'the url ' . (has_prefix($url, 'https://') ? 'does' : 'does not') . ' start with https://';
回答by viriathus
Also work:
还工作：
if (eregi("^http:", $url)) {
echo "OK";
}
