What does reg mean in php

Regular expression, also known as regular expression, regular expression, regular expression, regular expression, regular expression (English: Regular Expression, often abbreviated as regex, regexp or RE).

Regular expressions use a single string to describe and match a series of strings that match a certain syntax rule. In many text editors, regular expressions are often used to retrieve and replace text that matches a certain pattern.

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1. Character class

1. Character class: can match what it contains Any character of

eg: /[abc]/ matches any one of the letters "a", "b", and "c".

2. Negative character class: Define the negative character class through the "^" character, which matches all characters not included in square brackets. When defining a negative character class, put a "^" symbol as the first character inside the left bracket.

eg:/[^abc]/ matches all characters except "a", "b", and "c".

3. \s: Matches spaces, tabs and other Unicode whitespace characters;

. : Any character except newlines and other Unicode line terminators.

2. Repeat

1. {n,m} matches the previous item at least n times, but not more than m times;

2. {n,} matches the previous item n times or more;

3, {n} matches the previous item n times;

4, ? matches the previous item 0 or 1 times times;

5. * Matches the previous item 0 or more times;

6. Matches the previous item 1 or more times.

PS: Non-greedy Repeat---just follow the character to be matched with a question mark. "??" " ?" "*?"

3. Selection, grouping and referencing

1. "|" separates characters for selection

eg:/ab|cd|ef/ can match the string "ab", the string "cd", or the string "ef"

2, "()": circle The functions of parentheses: a. Combine individual items into subexpressions; b. Define subpatterns in a complete pattern; c. Allow reference to previous subexpressions at the end of the same regular expression.

3. "(?:" and ")" are grouped, but the characters matching the shuffling are not memorized

4. Specify the matching position

1. The reasonable position where the match occurs, the anchor of the regular expression.

2. "^" matches the beginning of the string

3. "$" matches the end of the string

4. Any regular expression can be used as an anchor point. If you add an expression between the symbols "(?=" and ")", it is a lookahead assertion.

5. Negative lookahead assertions "(?!" and ")"

5. Modifiers

#1. i is not case sensitive

2. g Global matching

3. m Matching in multiple lines

6. RegExp method

---- --- String method -------

1. search() returns the position of the first matching string;

2. repalce() performs retrieval and Replacement operation

If the regular expression sets the modifier g, then all strings matching the pattern in the source string will be replaced with the string specified by the second parameter; if there is no modifier g, Then only the first matching substring is replaced.

3. match() returns an array composed of matching results

-------RegExp method-------

4 , exec()

5, test()

========================== Some commonly used regular expressions ========================

1. Delete the blank characters in the first and last lines (including spaces, tabs, and other Unicode blanks characters)

var pattern = /(^\s*)|(\s*$)/g ;
"  abc  def  ".replace(pattern,""); //执行结果是："abc  def"

2. Strong password: The password is 8-12 characters, and contains uppercase letters, lowercase letters, numbers, and special characters

var strongRegExp = /^(?=.{8,12})(?=.*[A-Z])(?=.*[a-z])(?=.*[0-9])(?=.*\W).*$/g ;

3. Weak password: The password is 7- 12 digits, and contain any two of uppercase letters, lowercase letters, numbers, and no special characters

var mediumRegExp = /^(?=.{7,12})(((?=.*[A-Z])(?=.*[a-z]))|((?=.*[A-Z])(?=.*[1-9]))|((?=.*[a-z])(?=.*[1-9]))).*$/g ;

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