Strings and regular expressions in PHP

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1. String connectors
"." can directly connect two strings, or connect two string variables, or Connect strings and string variables

<?php$name="张先生";$a="欢饮您的到来";echo $name.$a;echo "\n";echo "李先生".$a;echo "\n";echo $name."欢迎你";?>

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2. Single quotes and double quotes of strings
Double quotes will output the value of the variable, and the variable name will be displayed directly within single quotes

<?php$message="PHP程序";echo "这是一个关于双引号和$message";echo "\n";echo &#39;这是一个关于单引号和$message&#39;;?>

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##3. Basic operations on strings
①Calculate the length of the string

<?php$str="这个字符串的长度";$length=strlen($str);if($length>20)    echo "字符串的长度不能大于20";else
    echo "此字符串的长度为$length";?>

②String word statistics str_word_count() function can only count English words. Will not work on Chinese characters

<?php$some="i am a girl,and you?";$some1="我是一个女孩，你呢？";echo str_word_count($some);echo "\n";echo str_word_count($some1);?>

③Interception of string substring substr (target string, starting position, interception length)
If they are all positive numbers, the integer at the starting position must be smaller than the integer of the intercepted length, otherwise the function value returns false
If the intercepted length is a negative number, it means that the target character is removed from the starting position. All characters except the length number of characters from the end of the string

<?php$some="create a substring of this string.";echo substr($some,0,11);echo "\n";echo substr($some,1,10);echo "\n";echo substr($some,1,-3);?>

④String substring replacement substr_replace(target string, Replacement string, starting position, replacement length)

<?php$some="i am a good girl";echo substr_replace($some,"beautiful",7,4);?>

⑤String search strstr (target string, need to find string)
If the function finds the character or string that needs to be found, it will return all the string contents from the first position where the string is found to the next

<?php$some="i have a Dream that i want to be a doctor.";$some1="我有一个想成为医生的梦想。";
echo strstr($some,"doctor");echo "\n";echo strstr($some,"that");echo "\n";echo strstr($some1,"成为");?>

4. Regular expression Concept: Regular expression is a method of expressing text or string according to certain specifications or models (commonly used for text matching operations)

①Form Brackets ([ ]) The string of characters within the square brackets are the characters that will be used for matching. For example, the regular expression [name] in square brackets means to find the letters n, a, m, e in the target string.

② hyphen (-) ([start-end] ) In many cases, it is not possible to list all characters one by one
[a-z] means matching any English lowercase character from a to z
[A-Za-z] means matching English uppercase and lowercase characters Any character from uppercase A to lowercase z
[0-9] means matching any decimal number from 0 to 9

③ period character (.) point The symbol character is a wildcard character in regular expressions, which represents all characters and numbers. For example, ".er" represents all three-character strings ending with er

④Qualifier ( ?{n,m})* The plus sign " " indicates that there is at least one character in front of it. For example, "9" means that the target string contains at least one 9
Asterisk "
" means that the preceding character is more than one or zero. For example, "y" means that the target string contains 0 or more than one y . The question mark "?" means that the character in front of it is one or zero. For example, "y?" means that the target string contains zero or one y
The braces "{n,m}" means that there are n or m characters in front of it. For example, "a{3,5}" means that the target string contains 3 or 5 a's. "a{3}" means that the target string contains 3 a's. "a{3,}" means that the target string contains at least 3 a
periods and asterisks can be used together, such as ".*" means matching any character

⑤ line locator (^ and $) The line locator is used to determine the position where the matching string will appear
If it appears at the beginning of the target string, use the symbol "^"; if it appears at the end of the target string appears, use the symbol "

means that 8895 can only appear at the end of the target string

and use it at the same time"^

" means that the target string should only contain a single character from a to z

⑥Exclude characters ([^]) For example, [^0-9] means that the target string contains any other characters "except" from 0-9

⑦Select the character (|)
Select the character to represent or. For example, com|cn|net means that the target string contains com or cn or net

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