PHP regular expression study notes_php skills

PHP regular expression study notes sharing:

1. Create a regular expression

$regex = '/d/i';
It's a bit like the first method in JavaScript, except here the word is a string.

2. Special characters in regular expressions

Special characters include: . * ? [ ^ ] $ ( ) { } = ! < > | : -

3. Functions in regular expressions

There are 8 methods, preg_match and preg_match_all, preg_replace and preg_replace_callback, preg_grep, preg_split, preg_last_error and preg_quote.

preg_match:

Perform a regular expression match

Returns the number of matches of pattern. Its value will be 0 times (no match) or 1 time, because preg_match() will stop searching for after the first match of .

$subject = "dd133aa2";
$pattern = '/\d+/';
preg_match($pattern, $subject, $matches, PREG_OFFSET_CAPTURE);
print_r($matches);

The above example code adds the parameter "PREG_OFFSET_CAPTURE". In this case, there will be one more offset number in $matches. For example, the following "2"

preg_match_all:

Perform a "global" regular expression match

Returns the complete number of matches (possibly 0), or FALSE if an error occurs.

$subject and $pattern in the code below are the same as above, the only difference is that preg_match is replaced by preg_match_all.

preg_match_all($pattern, $subject, $matches, PREG_OFFSET_CAPTURE);
print_r($matches);

The number of times returned is 2, and there are two matches. Look at the input in the $matches array, there are two. One more than the one above.

preg_replace:

Perform a regular expression search and replace

If subject is an array, preg_replace() returns an array, otherwise it returns a string.

If a match is found, the replaced subject is returned, otherwise the unchanged subject is returned. If an error occurs, NULL is returned.

In the code, $subject is an array with two sets of strings. Next, multiple numbers are replaced with uppercase "Z".

$subject = array("dd133aa2", "kk1ff3");
$pattern = '/\d+/';
$result = preg_replace($pattern, 'Z', $subject);
print_r($result);

preg_replace_callback:

Performs a regular expression search and replaces using a "callback"

Except that you can specify a callback instead of replacement to calculate the replacement string, the behavior of this function is equivalent to preg_replace() in other aspects, including the returned result.

The following code is also replaced with a capital "Z". The content of each $matches in the callback function is the commented part of the code. The first time is 133 and the second time is 2.

$subject = "dd133aa2";
$pattern = '/\d+/';
$result = preg_replace_callback($pattern, function($matches) {
    //$matches [0] => 133
    //$matches [0] => 2
    return 'Z';
   }, $subject);
print_r($result); 

preg_grep:

Returns array entries that match the pattern

Returns an array indexed using the key in the input as .

In the sample code below, I added "ddsdfd" to the $subject array, which does not contain numbers. When doing the matching, I filtered out the ones without numbers.

What $result2 prints is just the opposite. It prints the filtered one, but the key is still 2, not 0.

$subject = array("dd133aa2", "kk1ff3", "ddsdfd");
$pattern = '/\d+/';
$result = preg_grep($pattern, $subject);
$result2 = preg_grep($pattern, $subject, PREG_GREP_INVERT);
print_r($result);
print_r($result2);

preg_split：

通过一个正则表达式分隔字符串

返回一个使用 pattern 边界分隔 subject 后得到 的子串组成的数组。

下面代码中，我将$pattern中的表达式加了括号，为了在$result2中捕获到。

$subject = "dd133aa2cc";
$pattern = '/(\d+)/';
$result = preg_split($pattern, $subject);
$result2 = preg_split($pattern, $subject, null, PREG_SPLIT_DELIM_CAPTURE);
print_r($result);
print_r($result2);

preg_last_error：

返回最后一个PCRE正则执行产生的错误代码

preg_match('/(&#63;:\D+|<\d+>)*[!&#63;]/', 'foobar foobar foobar');
$result = preg_last_error();//PREG_BACKTRACK_LIMIT_ERROR 调用回溯限制超出
print_r($result);

preg_quote：

转义正则表达式字符

返回转义后的字符串。

下面的代码中，$subject中有两个需要转义的字符，“.”和“?”。

将$result打印出后是“dd\.a\?a2cc”，而在$result2中，多加了个参数“a”，这样的话“a”也会被转义，“dd\.\a\?\a2cc”

$subject = "dd.a?a2cc";
$result = preg_quote($subject);
$result2 = preg_quote($subject, 'a');
print_r($result);
print_r($result2);

4、模式修正符

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