What is a regular expression?
Regular expressions are objects that describe character patterns. Regular expressions are search patterns formed by a sequence of characters.
Syntax of regular expression: / regular expression body / modifier (optional)
For example:
const exp = /abc/i
/abc/i is a regular expression, where abc is a regular expression body (for regular matching), and i is a modifier, indicating that regular matching is case insensitive.
The meaning of this expression matches the string containing abc, where abc is not case sensitive.
const exp = /abc/i
console.log(exp.test('abc')) //If abc is included in the string, return true
console.log(exp.test('abcd')) //If abc is included in the string, return true
console.log(exp.test('Abcd')) //The string contains Abc and returns true
console.log(exp.test('bcd')) //abc is not included in the string, and false is returned
console.log(exp.test('ab2cd')) //abc is not included in the string, and false is returned
1. Modifier
| Modifier | effect |
|---|---|
| i | Perform case insensitive matching |
| g | Perform a global match (find all matches instead of stopping after the first match is found) |
| m | Perform multiline matching |
2. Square brackets
Square brackets are used to find characters in a range:
| expression | effect |
|---|---|
| [abc] | Find any character between square brackets. |
| [^abc] | Find any character that is not a character in square brackets. |
| [0-9] | Find any number from 0 to 9. |
| [a-z] | Find any character written from a to z. |
| [A-z] | Find any character from uppercase A to lowercase z. |
Note that square brackets are only used to find a character. If you want to find a string containing abc or 123, you can use square brackets and parentheses together:
let exp = /[(abc)(123)]/
console.log(exp.test('abcd')); //true
We can also use another method to achieve the same effect:
let exp = /(abc|123)/
console.log(exp.test('abcd')); //true
3. Metacharacter
Metacharacters are characters with special meanings
| Metacharacter | effect |
|---|---|
| ^ | Match the beginning of the string (when ^ appears in square brackets, it means matching characters outside the brackets) |
| $ | Match end of string |
| . | Finds a single character, except for line breaks and line terminations. |
| \w | Find numbers, letters, and underscores. |
| \W | Find non word characters. |
| \d | Find a number. |
| \D | Find non numeric characters. |
| \s | Find white space characters. |
| \S | Find non white space characters. |
| \b | Match word boundaries. |
| \B | Matches non word boundaries. |
| \0 | Find NULL character. |
| \n | Find line breaks. |
| \f | Find page feed. |
| \r | Find carriage return. |
| \t | Find tabs. |
| \v | Find vertical tabs. |
| \xxx | Find the character specified in octal number xxx. |
| \xdd | Finds the character specified in hexadecimal number dd. |
| \uxxxx | Finds Unicode characters specified in hexadecimal number xxxx. |
Bold metacharacters need us to remember that other metacharacters are not used frequently.
4. Quantifier
| classifier | describe |
|---|---|
| n+ | Matches any string that contains at least one n. |
| n* | Matches any string containing zero or more n's. |
| n? | Matches any string containing zero or one n. |
| n{X} | Matches a string containing a sequence of X n. |
| n{X,} | X is a positive integer. The previous pattern n matches when it occurs at least x times in a row. |
| n{X,Y} | X and Y are positive integers. The previous pattern n appears continuously at least x times and matches at most Y times. |
| ?=n | Matches any string followed by the specified string n. |
| ?!n | Matches any string that is not immediately followed by the specified string n. |
Note that all n here can be a character or a subexpression.
5. Application of regular expressions
5.1 matching telephone number:
The format is ******* - ****** or ******, where * represents a number
let exp1 = /^\d{3}-\d{4}(-\d{4})?$/
let exp2 = /^\d{3}(-\d{4}){1,2}$/
console.log(exp1.test('176-5465-8777')); //true
console.log(exp2.test('123-4567')); //true
console.log(exp1.test('17654658777')); //false
console.log(exp2.test('1234567')); //false
5.2 matching date:
The format is year / month / day or year month day
let exp = /^(\d+\/\d{1,2}\/\d{1,2})|(\d+-\d{1,2}-\d{1,2})$/
console.log(exp.test('2019-02-24')); //true
console.log(exp.test('2019/02/24')); //true
console.log(exp.test('2019-02/24')); //false
Note that we cannot let exp = /^\d+[/-]\d{1,2}[/-]\d{1,2} $/. This time, the result of exp.test('2019-02/24 ') is true