Explanation and application of regular expression look-ahead

explain

Lookaround, also known as Lookaround, is a type of Zero-Length Assertions and has 4 syntaxes:

grammar	Chinese name	English name
(?=regex)	positive look-ahead	Positive lookahead
(?!regex)	negative look-ahead	Negative lookahead
(?<=regex)	positive look	Positive lookbehind
(?<!regex)	negative look	Negative lookbehind

Simpler explanation:

• Positive : Match regex
• Negative : no match regex
• Look ahead : look forward (right, forward) (look ahead)
• Backward look: look backward (left, reverse) (pre-look)
• Positive look-ahead : first see if the front (right) matches regex , but don't move forward
• Negative look -ahead: first see if the front (right) does not match regex , but do not move forward
• Positive retrospective: look back to see if the rear (left) matches regex
• Negative look-back: look back to see if the rear (left) does not match regex

Why is the right side forward and forward and the left side reverse and backward?

Because the regex is matched from left to right , just like a person walking, then looking forward (looking forward) is right, forward, and looking backward (backward) is left and reverse , pay special attention not to confuse it with prefix and suffix

application

1. Determine whether it contains but not limited to uppercase letters, lowercase letters and numbers, and cannot contain 114514 , the length is 8~32 characters

 /^(?=.*[A-Z])(?=.*[a-z])(?=.*\d)(?!.*114514).{8,32}$/.test('[e-mail]')
// true

2、 80 、 82 、 84 、 86 、 88

 /^(?!8[02468])\d+$/.test('81789110')
// true

3. Match a string of pure numbers in a string separated by whitespace

 '123 abc   1st\r\n4   1970\'s 56 @10086 789'.match(/(?<!\S)\d+(?!\S)/g)
// ['123', '4', '56', '789']
'123 abc   1st\r\n4   1970\'s 56 @10086 789'.match(/(?<=^|\s)\d+(?=\s|$)/g)
// ['123', '4', '56', '789']

4. Only remove commas sandwiched in numbers

 'And then, I have 1,003,334, you have 996,6,6,6'.replace(/(?<=\d),(?=\d)/g, '')
// 'And then, I have 1003334, you have 996666'

5. Add grouping commas to numbers

 'And then, I have 1003334, you have 996666'.replace(/(?<=\d)(?=(?:\d{3})+(?!\d))/g, ',')
// 'And then, I have 1,003,334, you have 996,666'

6. Determine whether it contains only letters, but not lowercase vowels

 /^(?:(?![aeiou])[A-Za-z])+$/g.test('src')
// true
/^(?=[A-Za-z]+$)[^aeiou]+$/g.test('src')
// true

7. Determine whether it is a letter or number or letter + number, cannot be empty

 /^(?=.)[a-z]*\d*$/i.test('Add1')
// true

8. Match a number with a length of 11, be careful not to be a substring of other numbers

 `name:123456789
id:11012345678910
tel:[phone]
name:abc11111111111
id:888888888888
tel:11966`.match(/(?<!\d)\d{11}(?!\d)/g)
// ['12345678910', '11111111111']

9. If the protocol prefix is missing, add http:// , ignore blank lines

 String.raw`segmentfault.com
//bing.com
\baidu.com

127.0.0.1
ftp://127.0.0.1

https://google.com

`.replace(/^(?![a-z]+:)[\\/]*(?=.)/gim, 'http://')
/*
http://segmentfault.com
http://bing.com
http://baidu.com

http://127.0.0.1
ftp://127.0.0.1

https://google.com

*/