🔥 7 regular expressions I don't want to run into again

Maybe you and I have never met, but we may not have met. I am a head fish.

preface

Job-hopping is the peak these days! Several colleagues jumped byte, Meituan, Ali, salary rose a big wave, envy my heart itch, and how the gap between people is so big, can not help but feel that he is a garbage!

The chance to ask a wave of these giant face what topic, including source code analysis, algorithm, computer networks, JS, front-end engineering and so on, but there is a small point seems everybody is met, is a regular expression, although said that they have got the offer, but regular matching answer is not very good, can’t help reminds me of before the interview, And the damn regex lost.

Today, Chubbyhead tried to scoop up the regular expression related questions common in the front-end interview, hoping to directly pass the point of the regular expression later

1.# Number price split in thousandths

Change 123456789 to 123,456,789

This problem is estimated that we often meet in the interview and work, the frequency is relatively high.

Regular results

'123456789'.replace(/ (? ! (^)? =(\d{3})+$)/g.', ') / / 123456789
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Added small quartile support

The analysis process

The question probably means:

Add a comma before every third digit from the back
Do not start with a comma (e.g.,123 does not end with 123)

Does it fit (? What’s the rule of p? P can represent every three digits, and the comma to be added is exactly where (? =p) matched position.

Step one, try to get the first comma out of it


let price = '123456789'
let priceReg = / (? =\d{3}$)/

console.log(price.replace(proceReg, ', ')) // 123456,789
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Step two, get all the commas out

To get all the commas out, the main problem is how to represent groups of three numbers, multiples of three. We know that regular brackets can turn a p pattern into a small whole, so using the properties of brackets, we can write it like this


let price = '123456789'
let priceReg = / (? =(\d{3})+$)/g

console.log(price.replace(priceReg, ', ')) / /, 123456789

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Step three, remove the first comma,

It is not enough to remove the first comma from the list. Is there a knowledge that fits this scenario? That’s right. ! P), that’s it, the combination of the two is to add a comma before every three digits, but this position cannot be the first.


let price = '123456789'
let priceReg = / (? ! (^)? =(\d{3})+$)/g

console.log(price.replace(priceReg, ', ')) / / 123456789
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2.# Phone number 3-4-4 split

Change the phone number 18379836654 to 183-7983-6654

Form collection scenarios are often encountered in mobile phone formatting

Regular results

let mobile = '18379836654' 
let mobileReg = / (? =(\d{4})+$)/g 

console.log(mobile.replace(mobileReg, The '-')) / / 183-7983-6654

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The analysis process

With the thousandths of the number above, it would be much easier to do the problem, which is to find the position from back to front:

Every four digits in front of the position, and replace this position with –


let mobile = '18379836654'
let mobileReg = / (? =(\d{4})+$)/g

console.log(mobile.replace(mobileReg, The '-')) / / 183-7983-6654

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3.# hump strings

As follows, change the corresponding string to hump

1. foo Bar => fooBar

2. foo-bar---- => fooBar

3. foo_bar__ => fooBar

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Regular results

const camelCase = (string) = > {
  const camelCaseRegex = /[-_\s]+(.) ? /g

  return string.replace(camelCaseRegex, (match, char) = > {
    return char ? char.toUpperCase() : ' '})}Copy the code

The analysis process

Analyze the pattern of the problem

It comes before every wordZero or more- The blank space _Such as,Foo,--foo,__FOO,_BAR, Bar)
- The blank space _It may not be followed by anything like (__,--)

const camelCase = (string) = > {
  / / note (.). ? Here? It's in order to satisfy condition two
  const camelCaseRegex = /[-_\s]+(.) ? /g

  return string.replace(camelCaseRegex, (match, char) = > {
    return char ? char.toUpperCase() : ' '})}console.log(camelCase('foo Bar')) // fooBar
console.log(camelCase('foo-bar--')) // fooBar
console.log(camelCase('foo_bar__')) // fooBar

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4.# Convert the first letter of the string to uppercase and the rest to lowercase

For example, hello World becomes Hello World

Regular results


const capitalize = (string) = > {
  const capitalizeRegex = / (? :^|\s+)\w/g

  return string.toLowerCase().replace(capitalizeRegex, (match) = > match.toUpperCase())
}

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The analysis process

Find the first letter of the word and convert it to a capital letter. The word may be preceded by a beginning or multiple Spaces.


const capitalize = (string) = > {
  const capitalizeRegex = / (? :^|\s+)\w/g

  return string.toLowerCase().replace(capitalizeRegex, (match) = > match.toUpperCase())
}

console.log(capitalize('hello world')) // Hello World
console.log(capitalize('hello WORLD')) // Hello World

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5.# obtain the URL query parameter by name

Regular results

const getQueryByName = (name) = > {
  const queryNameRegex = new RegExp(`? [&]${name}) = (/ ^ & * (& | $) `)
  const queryNameMatch = window.location.search.match(queryNameRegex)
  // Generally through decodeURIComponent decoding processing
  return queryNameMatch ? decodeURIComponent(queryNameMatch[1) :' '
}

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The analysis process

The parameter name= on the URL query may be the position of the front-end bighead fish

Following the question mark? Name = bighead &sex=boy
In the last position? Sex =boy&name= front end bighead fish
Between one and two? Sex =boy&name= bighead &age=100

So you only have to deal with three places and you can basically get it by regex

Name can only be preceded by? Or &
The value of a value can be anything except an ampersand
Value can only be followed by an ampersand or an end position


const getQueryByName = (name) = > {
  const queryNameRegex = new RegExp(`? [&]${name}(= (/ ^ & *)? : & ` | $))
  const queryNameMatch = window.location.search.match(queryNameRegex)
  // Generally through decodeURIComponent decoding processing
  return queryNameMatch ? decodeURIComponent(queryNameMatch[1) :' '
}
// 1
// https://juejin.cn/?name= front end bighead &sex=boy
console.log(getQueryByName('name')) // Front end bighead fish

// 2. Name is at the end
// https://juejin.cn/?sex=boy&name= front-end bighead fish
console.log(getQueryByName('name')) // Front end bighead fish


// 2. Name is in the middle
// https://juejin.cn/?sex=boy&name= bighead &age=100
console.log(getQueryByName('name')) // Front end bighead fish


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6.# Extract consecutive repeated characters

The original coin security problem will have repeated characters extracted, such as 12323454545666, extracted [’23’, ’45’, ‘6’]

Regular results

const collectRepeatStr = (str) = > {
  let repeatStrs = []
  const repeatRe = /(.+)\1+/g
  
  str.replace(repeatRe, ($0, $1) = >{$1 && repeatStrs.push($1)})return repeatStrs
}

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The analysis process

There are several key messages in the question

Consecutive repeated characters
There is no limit to the length of consecutive repeated characters (e.g. 23, 45 are two digits, and 6 is one digit)

So what is continuous repetition?

11 is a continuous repeat, 22 is a continuous repeat, 111 is certainly a continuous repeat. That is, some character X must be followed by X, which is called continuous repetition. If we know that X is exactly 1, then PI over 11 plus PI over 1 would match, but the point is that we don’t know what X is here, so what do we do? .

This problem can be easily solved by using the re knowledge of backreferences.

The first step is to write out a re that represents a single character repetition

// X is available here. All the characters are referenced in parentheses, followed by the reverse application \1, which reflects the meaning of continuous repetition
let repeatRe = / (.). \ 1 /

console.log(repeatRe.test('11')) // true
console.log(repeatRe.test('22')) // true
console.log(repeatRe.test('333')) // true
console.log(repeatRe.test('123')) // false

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Second, write out the re that represents n character repetitions

The quantifier + in parentheses is used to indicate n repeated characters because it is not certain whether to match 11 or 45, 45, and the backreference itself can be more than one, such as 45, 45, 45


let repeatRe = / (+) \ + / 1

console.log(repeatRe.test('11')) // true
console.log(repeatRe.test('22')) // true
console.log(repeatRe.test('333')) // true
console.log(repeatRe.test('454545')) // true
console.log(repeatRe.test('124')) // false

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The third step is to extract all consecutively repeated characters


const collectRepeatStr = (str) = > {
  let repeatStrs = []
  const repeatRe = /(.+)\1+/g
  // Most of the time replace is not used for replacement, but for data extraction
  str.replace(repeatRe, ($0, $1) = >{$1 && repeatStrs.push($1)})return repeatStrs
}


console.log(collectRepeatStr('11')) / / / "1"
console.log(collectRepeatStr('12323')) / / / "23"
console.log(collectRepeatStr('12323454545666')) / / / "23", "45", "6"]

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# implement a trim function

Here we have two ways of removing the beginning and end of a string

Regular results

/ / remove the blank space method const trim = (STR) = > {return STR. Replace (/ ^ | \ \ s * * $s/g, "')} / / extraction method of the space const trim = (STR) = > {return STR. Replace (/ ^ \ s * (. *?) \s*$/g, '$1') }Copy the code

The analysis process

When we first look at the problem, the first thing that comes to our mind is to delete the blank part and keep the non-blank part, but we can also go another way, we can also extract the non-blank part, regardless of the blank part. Next we write the two trim method implementations

Method one, remove the space method


const trim = (str) = > {
  return str.replace(/^\s*|\s*$/g.' ')}console.log(trim('Front End Bighead')) // Front end bighead fish
console.log(trim('Front End Bighead')) // Front end bighead fish
console.log(trim('Front End Bighead')) // Front end bighead fish
console.log(trim('Front End Bighead')) // Front end bighead fish

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Method two, extract non – space method


const trim = (str) = > {
  return str.replace(/^\s*(.*?) \s*$/g.'$1')}console.log(trim('Front End Bighead')) // Front end bighead fish
console.log(trim('Front End Bighead')) // Front end bighead fish
console.log(trim('Front End Bighead')) // Front end bighead fish
console.log(trim('Front End Bighead')) // Front end bighead fish

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At the end

Here are 7 regular expression interview questions that you will encounter in your interview with a few other friends.

I hope I can share practical, basic and advanced knowledge points with you all the time.

I look forward to your attention in nuggets: front end bighead fish, you can also find me in the public number: front end bighead fish.

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🔥 7 regular expressions I don’t want to run into again

preface

1.# Number price split in thousandths

Regular results

The analysis process

2.# Phone number 3-4-4 split

Regular results

The analysis process

3.# hump strings

Regular results

The analysis process

4.# Convert the first letter of the string to uppercase and the rest to lowercase

Regular results

The analysis process

5.# obtain the URL query parameter by name

Regular results

The analysis process

6.# Extract consecutive repeated characters

Regular results

The analysis process

# implement a trim function

Regular results

The analysis process

At the end

🔥 7 regular expressions I don’t want to run into again

preface

1.# Number price split in thousandths

Regular results

The analysis process

2.# Phone number 3-4-4 split

Regular results

The analysis process

3.# hump strings

Regular results

The analysis process

4.# Convert the first letter of the string to uppercase and the rest to lowercase

Regular results

The analysis process

5.# obtain the URL query parameter by name

Regular results

The analysis process

6.# Extract consecutive repeated characters

Regular results

The analysis process

# implement a trim function

Regular results

The analysis process

At the end

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