Regular expressions (2)

Regular expressions are pattern matching, or matching location, or matching character

1. What position?

Position is a position between adjacent characters, such as hello, is the location where the arrow

 

2, how to match the position?

 

 In ES5, a total of six characters: 

1 ^   beginning matching
 2  $ matches the end
 . 3  \ B word boundary
 . 4  \ B \ B opposite, non-word boundary
 . 5 (? = P) a subpattern p, i.e. in front of the p-position
 . 6 (?! P) (? = p) the opposite

Here we introduced one by one these characters

And $ 2.1 ^

^ (Caret) matches the beginning, the beginning of the match line in multi-line matching.

$(Dollar sign) the end of the match, matching end of the line in multi-line matching.

For example, we put at the beginning and end of the string with a "#" with (the position can be replaced with characters!):

1 var result = "hello".replace(/^|$/g, '#');
2 console.log(result); 
3 // => "#hello#"

2.2 \ b and \ B

\bIt is a word boundary, that is, specific \wand \Wlocation between, but also \wand ^position between, but also \wand $position between.

For example, a file name is "[JS] Lesson_01.mp4" are \bas follows:

1 var result = "[JS] Lesson_01.mp4".replace(/\b/g, '#');
2 console.log(result); 
3 // => "[#JS#] #Lesson_01#.#mp4#"

Such as the example above, all \Breplaced by "#":

1 var result = "[JS] Lesson_01.mp4".replace(/\B/g, '#');
2 console.log(result); 
3 // => "#[J#S]# L#e#s#s#o#n#_#0#1.m#p#4"

2.3 (? = P) and (?! P)

(?=p)Which pis a sub-mode, i.e., pthe front position.

For example (?=l), indicates that the 'l' character positions, for example:

1 var result = "hello".replace(/(?=l)/g, '#');
2 console.log(result); 
3 // => "he#l#lo"

And (?!p)that is (?=p)the opposite of meaning, such as:

1 var result = "hello".replace(/(?!l)/g, '#');
2 
3 console.log(result); 
4 // => "#h#ell#o#"

3. Case in point:

3.1 Digital micrometer separator notation

Such as the "12345678", becomes "12,345,678."

step1: come up with a final comma

var result = "12345678".replace(/(?=\d{3}$)/g, ',')
console.log(result); 
// => "12345,678"

Come up with all the commas

1 var result = "12345678".replace(/(?=(\d{3})+$)/g, ',')
2 console.log(result); 
3 // => "12,345,678"

3.2 verification code issues

6-12 password length, digital, lowercase and uppercase letters, but must include at least two characters.

3.2.1 If you do not consider contain 2 characters can easily write

1  var reg = / ^ [0-9A-Za-z] {6,12} $ /;

3.2.2 to determine whether to include certain characters

Assumptions must contain lowercase letters how to do? Can (? =. * [0-9])

* Matches any single character times

. *? Matches the condition is satisfied only once

var reg = 0909AZaz612;

3.2.3 colleagues include specific two character

var reg = 09az09AZaz612;

3.2.4 contain at least two characters

var reg = 09az09AZ . * [az]) (? =. * [AZ])) ^ [0-9A-Za-z] {6,12} $ /;

3.2.5 Another solution: contains at least two characters that can not be all digital, can not be all lowercase letters, not all uppercase letters

var reg = / (?! ^ [0-9] {6,12} $) (?! ^ [z] {6,12} $) (?! ^ [AZ] {} $ 6.12) ^ [ 0-9a-Za-z] {} $ 6.12 /;