RegEx - Change string with format

Question

I'm still new in regex and still learning. But struggle to achieve this one.

What I want from here is to change a specific number with this format:

(space)(number)(space) or (space)(number)(word)

This is my current code:

let initial = "Sample 50 This 500是是 50是 of 50是. Sample 450是是";
let regex = new RegExp("\\b50|\\b50\\D", "g");
let result = initial.replace(regex, "9");
console.log(result);

So from this string:

Sample 50 This 500是是 50是 of 50是. Sample 450是是

The result should be this one:

Sample 9 This 500是是 9是 of 9是. Sample 450是是

But my current result is this (500 also changed):

Sample 9 This 90是是 9是 of 9是. Sample 450是是

Is this possible? Thanks for the help.

CertainPerformance · Accepted Answer · 2019-08-11 04:56:15Z

1

The problem with your current pattern is that the left-side alternation \b50 will match 50 even if there are more adjacent digits immediately to the right of the 50 - which you want to make sure there aren't.

After matching the number, negative lookahead for a digit:

\b50(?!\d)

replace with 9:

const input = "Sample 50 This 500是是 50是 of 50是. Sample 450是是";
console.log(
  input.replace(
    /\b50(?!\d)/g,
    '9'
  )
);

Note that since you aren't constructing the pattern dynamically, there's no need for new RegExp - use a regex literal instead, so you don't have to double-escape the backslashes (which make it harder to read).

answered Aug 11, 2019 at 4:56

CertainPerformance

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2 Comments

Pepeng Hapon Over a year ago

Hi @CertainPerformance, your solution works great and It has a readable code. I have noted the additional link you've added. Many thanks for the info. One quesiton, where can I find to read the docu about this one: !\d 'cause what I know is that the counterpart of \d is \D

CertainPerformance Over a year ago

(?! is the syntax for negative lookahead. Eg foo(?!x) will match foo in fooa, but not foo in foox. Negative lookahead for \d and matching \D like you're doing are pretty similar - both ensure that what was matched before isn't followed by a digit, but \D will mean that the non-digit character is matched as well (eg all characters in50是 will be matched, but you only want to match the 50 part, because that's the only part you want to replace).

Code Maniac · Accepted Answer · 2019-08-11 05:08:33Z

1

You can use this

(\D|^)50(\D|$)

(\D|^) - Match Non digit character or start of string ( group1 -> $1 )
50 - Match 50
(\D|$) - Match Non digit character or end of string ( group2 -> $2 )

let initial = "50Sample 50 This 500是是 50是 of 50是. Sample 450是是50";
let regex = /(\D|^)50(\D|$)/g
let result = initial.replace(regex, "$19$2");
console.log(result);

edited Aug 11, 2019 at 5:08

answered Aug 11, 2019 at 5:01

Code Maniac

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2 Comments

Pepeng Hapon Over a year ago

Hi @Code Maniac, your solutions works! But CertainPerformance made the first answer. But I upvoted your answer. One question though, what is the meaning of "$19$2"? Appreciated your help. Thank you!

Code Maniac Over a year ago

@PepengHapon always happy to help, $1 -> back reference to group one, $2 back reference to group two, so this actually means place captured value of group one 9 place captured value of group 2

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