Detailed explanation of regular expressions in PHP (code examples)

Objectives of this article:

1. Definition of regular expressions

2. Several basic grammars of regular expressions

(1) Definition of regular expressions

Regular expression is a logical formula for operating strings, which is to combine some specific characters into a regular string

For example:

The above code The '/abc123/' in is a regular expression. We can see from it that /abc123/ is a string composed of characters and numbers, but these characters have their own special meanings, such as /abc123/ The rule of this regular expression is that the string starts with abc123. If any string matches this rule, it will match this expression.

(2) Several basic syntaxes of regular expressions

1. The regular matching pattern uses delimiters and metacharacters. The delimiter can be any character other than numbers, non-backslashes, and non-spaces. Common delimiters such as forward slash (/), hash symbol (#) and negation symbol (~)

For example:

/hello world/ The meaning of the expression Yes: The string starts with hello world
#^[0-9]$# The expression means: Match the numbers 0-9
~hello~ The expression means: The string contains hello

Let’s test it with code

Example 1、

/hello world/ The expression means: The string starts with hello world

'; } ?>

The running result is as follows:

This string conforms to this rule/hello world/

Change a string to see Next, do not start with hello world

'; } ?>

The running result is:

Blank

Example 2,

#^[0-9]$# The meaning of the expression is: matching the numbers 0-9

'; } ?>

The running result is:

The The string conforms to this rule"#^[0-9]$

#Change the code and change the string to a number greater than 9 and see if it runs

'; }else{ echo '该字符串不符合这个规则"#^[0-9]$#
'; } ?>

The result is:

This string does not comply with this rule"#^[0-9]$

Example 3,

~hello~ The meaning of the expression is: the string contains hello

The specific code is as follows:

The running result is:

The The string conforms to this rule: ~hello~

Now change the test string to not contain hello

The specific code is as follows:

The running result is:

This string does not comply with this rule: ~hello~

It can be seen that:

1, / means the beginning

2 , ^ means starting with the character after ^

3, $ means ending with the character before $

4, ~ means containing the meaning

2, if If the pattern contains a delimiter, the delimiter needs to be escaped with a backslash (\).

For example:

/https:\/\/www./means starting with https://www.

The specific code As follows:

      
The running result is:
      

       
This string conforms to this rule: /https:\/\/www./
      
      
Try Change the string to not start withhttps://www. and see
      

      
The running result is:
      

       
This string does not comply with this rule: /https:\/\/www./
      
      
3. If the pattern contains a lot of delimiting characters, it is recommended to use other characters as delimiters, or you can use preg_quote to escape. .
      
Example 1,
      

      
The running result is:
      

       
Warning: preg_match(): Unknown modifier '/' in D:\E-class\class-code\classing\index.php on line 7
This string does not comply with this rule:/https://www.baidu.com/a/b/index.html/
      
      
So you cannot write directly at this time/Either escape as above, or proceed as follows
      
The specific code is as follows:
      

      
The running result is:

      

       
This string conforms to this rule:/https://www.baidu.com/a/b/index.html/
      
      
4. Pattern modifiers can be used after the delimiter. Pattern modifiers include: i, m, s, etc.
      
Summary:

      
1. i means the size can be ignored Write
      
2. m means multi-line matching
      
3. If this modifier is set, the dot metacharacter (.) in the pattern matches all characters, including newline characters. Without this setting, newline characters are not included.
      
Case 1,
      
Practice goals:
      
1. i means that case can be ignored
      

      
The running result is:

      

       
The string conforms to this rule:/ABc/i
      
      
Case 2,
      
Practice goal:
      
1. m means multi-line matching
      
The specific code is as follows:
      

      
The running result is:
      

       
This string conforms to this rule: / chinese/m, the matching result is: Array ( [0] => Array ( [0] => chinese [1] => chinese ) )
      
      
What should be noted here is that Use preg_match_all otherwise preg_match will only match one line
      
接下来我们运行下效果
      

      
运行结果为：

      

       
该字符串符合这个规则：/chinese/m,匹配结果为：Array ( [0] => chinese )
      
      
其实/m在此也算多此一举，因为preg_match_all就是表示多行匹配了
      

      
运行结果其实是一样的，结果为：
      

       
该字符串符合这个规则,匹配结果为：Array ( [0] => Array ( [0] => chinese [1] => chinese ) )
      
      
只是要知道m表示多行匹配的意思
      
案例三、
      
实践目标：
      
1、如果设定了此修正符，模式中的圆点元字符（.）匹配所有的字符，包括换行符。没有此设定的话，则不包括换行符。
      
具体代码如下：
      

      
运行结果如下：

      

       
该字符串符合这个规则,匹配结果为：Array ( [0] => Array ( [0] => chinese [1] => chinese g ) )
      
      
说明第一个chinese 后面的字符是换行也匹配到了，这说明了s的意思就是.要包含换行符，接下来
      
我们去掉s，看下最终的结果
      

      
运行结果如下：
      

       
该字符串符合这个规则,匹配结果为：Array ( [0] => Array ( [0] => chinese g ) )
      
      
说明此刻只匹配到一个了，因为.不包含换行符，所以第一个chinese没有匹配到
      
总结：
      
本文主要讲解了
      
1、正则表达式的定义
      
2、正则表达式的几个基本语法
      
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