PHP regular expression introductory tutorial [translated], regular expression introductory tutorial_PHP tutorial

PHP regular expression introductory tutorial [translated], regular expression introductory tutorial

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​ ​ Introduction ​ Regular expressions should be used frequently in development. Nowadays, many development languages ​​​​have regular expression applications, such as javascript, java, .net, php, etc. Today I will share my understanding of regular expressions with If you are nagging, please tell me if I am inappropriate! ​ Need-to-know terms – How much do you know about the following terms? ​ Δ delimiter Δ Character field Δ modifier Δ qualifier Δ caret Δ wildcard (forward preview, reverse preview) Δ back reference Δ Lazy matching Δ Comment Δ Zero character width ​ Positioning ​ When do we use regular expressions? It is not enough to use regular expressions for all character operations. PHP uses regular expressions in some aspects, which actually affects efficiency. When we encounter complex text data parsing, regular expressions are a better choice. ​ Advantages ​ Regular expressions can improve work efficiency when processing complex character operations, and also save your code to a certain extent. ​ Disadvantages ​ When we use regular expressions, complex regular expressions will increase the complexity of the code and make it difficult to understand. So sometimes we need to add comments inside regular expressions. ​ Common mode

​ ¤ Delimiter, usually "/" is used as the beginning and end of the delimiter, but "#" can also be used. When to use "#"? Usually when there are a lot of "/" characters in your string, because such characters need to be escaped during regular expressions, such as uri. The code using the "/" delimiter is as follows.

$regex = '/^http://([w.] )/([w] )/([w] ).html$/i'; $str = 'http://www.youku.com/show_page/id_ABCDEFG.html'; $matches = array();   if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

$matches[0] in preg_match will contain the string matching the entire pattern.

The code using the "#" delimiter is as follows. At this time, "/" will not be escaped!

$regex = '#^http://([w.] )/([w] )/([w] ).html$#i'; $str = 'http://www.youku.com/show_page/id_ABCDEFG.html'; $matches = array();   if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

¤ Modifier: used to change the behavior of regular expressions.

The last one we see in ('/^http://([w.] )/([w] )/([w] ).html/i') i" is the modifier, which means ignoring case, and another one we often use is "x" which means ignoring spaces.

Contribute code:

$regex = '/HELLO/'; $str = 'hello word'; $matches = array();   if(preg_match($regex, $str, $matches)){     echo 'No i:Valid Successful!',"n"; }   if(preg_match($regex.'i', $str, $matches)){     echo 'YES i:Valid Successful!',"n"; }

¤ Character field: [w] The part expanded with square brackets is the character field.

¤ Qualifier: such as [w]{3,5} or [w]* or [w]. The symbols after [w] all represent qualifiers. The specific meaning is now introduced.

{3,5} means 3 to 5 characters. {3,} is more than 3 characters, {,5} is up to 5 characters, and {3} is three characters.

* represents 0 to multiple

means 1 to more.

¤ caret

^:

& gt; placed in the character domain (such as: [^w]) indicate the negative (excluding meaning) - "reverse selection"

                                                                                                                                                                                            can be placed before the expression to start with the current character. (/^n/i, means starting with n).

Note, we often call "" "escape character". Used to escape some special symbols, such as ".", "/"

Wildcards (lookarounds): Assert the presence or absence of certain characters in certain strings! ​ There are two types of lookarounds: lookaheads (forward lookup ?=) and lookbehinds (reverse lookup?<=). > Format: Positive lookup: (?=) corresponding to (?!) means negative meaning Reverse pre-lookup: (?<=) The corresponding (? $regex = '/(?<=c)d(?=e)/'; /* d 前面紧跟c, d 后面紧跟e*/ $str = 'abcdefgk'; $matches = array(); if(preg_match($regex, $str, $matches)){ var_dump($matches); } echo "n";

Negative meaning:

$regex = '/(?<!c)d(?!e)/'; /* d 前面不紧跟c, d 后面不紧跟e*/ $str = 'abcdefgk'; $matches = array(); if(preg_match($regex, $str, $matches)){ var_dump($matches); } echo "n";

>Character width: zero Verify zero character codes

$regex = '/HE(?=L)LO/i'; $str = 'HELLO'; $matches = array();   if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

Cannot print the result!

$regex = '/HE(?=L)LLO/i'; $str = 'HELLO'; $matches = array();   if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

Can print out the results!

Explanation: (?=L) means HE is followed by an L character. However, (?=L) itself does not occupy a character and must be distinguished from (L), which itself occupies one character.

​ capture data ​ Groupings without specifying a type will be retrieved for later use. > indicates that the type refers to the wildcard character. Therefore, only those without question marks at the beginning of the parentheses can be captured.

> References within the same expression are called backreferences. > Calling format: number (such as 1).

$regex = '/^(Chuanshanjia)[ws!] 1$/';    $str = 'Chuanshanjia thank Chuanshanjia'; $matches = array();   if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

> Avoid capturing data Format:(?:pattern) Advantages: It will keep the number of effective back references to a minimum and the code will be clearer. ​ >Named capturing group Format: (?P) Calling method (?P=group name)

$regex = '/(?P<author>chuanshanjia)[s]Is[s](?P=author)/i'; $str = 'author:chuanshanjia Is chuanshanjia'; $matches = array();   if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

Run results

​

Lazy matching (remember: two operations will be performed, please see the principle part below)

Format: Qualifier?

Principle: "?": If there is a qualifier in front of it, the smallest data will be used. For example, "*" will take 0, and " " will take 1. If it is {3,5}, 3 will be taken.

Look at the following two codes first:

Code 1.

<?php $regex = '/heL*/i'; $str = 'heLLLLLLLLLLLLLLLL'; if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

Result 1.

Code 2

<?php $regex = '/heL*?/i'; $str = 'heLLLLLLLLLLLLLLLL'; if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

Result 2

Code 3, use " "

<?php $regex = '/heL ?/i'; $str = 'heLLLLLLLLLLLLLLLL'; if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

Result 3

Code 4, use {3,5}

<?php $regex = '/heL{3,10}?/i'; $str = 'heLLLLLLLLLLLLLLLL'; if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

Result 4

​ ​

Comments on regular expressions ​ Format: (?# comment content) Purpose: Mainly used for complex annotations ​ Contribution code: It is a regular expression used to connect to MYSQL database

$regex = '/     ^host=(?<!.)([d.] )(?!.)                 (?#主机地址) |     ([w!@#$%^&*()_ -] )                       (?#用户名) |     ([w!@#$%^&*()_ -] )                       (?#密码) (?!|)$/ix';   $str = 'host=192.168.10.221|root|123456'; $matches = array();   if(preg_match($regex, $str, $matches)){     var_dump($matches); }   echo "n";

​ special characters ​ ​

特殊字符	解释
*	0到多次
	1到多次还可以写成{1,}
?	0或1次
.	匹配除换行符外的所有单个的字符
w	[a-zA-Z0-9_]
s	空白字符(空格，换行符，回车符）[tnr]
d	[0-9]

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