javascript replace last occurrence of string

Question

I've read many Q&As in StackOverflow and I'm still having a hard time getting RegEX. I have string 12_13_12.

How can I replace last occurrence of 12 with, aa.

Final result should be 12_13_aa.

I would really like for good explanation about how you did it.

@GintasK no.. it wont always contain 2 digits and so as the other pair of numbers — Vera Gavriel
– Vera Gavriel, Commented Jul 23, 2013 at 11:18
'12_13_12'.replace(/12([^1][^2])*$/, 'aa') // 12 followed by any not 12 pattern — TheOne
– TheOne, Commented Jan 23, 2015 at 17:01

Casimir et Hippolyte · Accepted Answer · 2013-07-23 11:35:34Z

25

You can use this replace:

var str = '12-44-12-1564';
str = str.replace(/12(?![\s\S]*12)/, 'aa');
console.log(str);

explanations:

(?!            # open a negative lookahead (means not followed by)
   [\s\S]*     # all characters including newlines (space+not space)
               # zero or more times
   12
)              # close the lookahead

In other words the pattern means: 12 not followed by another 12 until the end of the string.

answered Jul 23, 2013 at 11:35

Casimir et Hippolyte

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2 Comments

Ro Yo Mi Over a year ago

+1 for using a regex solution and adding a detailed explanation.

tim-we Over a year ago

(plus one) for not requiring the search string to be at the end ($)

Gintas K · Accepted Answer · 2013-07-23 11:20:32Z

17

newString = oldString.substring(0,oldString.lastIndexOf("_")) + 'aa';

answered Jul 23, 2013 at 11:20

Gintas K

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Comments

anubhava · Accepted Answer · 2013-07-23 17:26:55Z

3

Use this String.replace and make sure you have end of input $ in the end:

repl = "12_13_12".replace(/12(?!.*?12)/, 'aa');

EDIT: To use a variable in Regex:

var re = new RegExp(ToBeReplaced);
repl = str.replace(re, 'aa');

edited Jul 23, 2013 at 17:26

answered Jul 23, 2013 at 11:17

anubhava

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9 Comments

Gintas K Over a year ago

it was just an example

Vera Gavriel Over a year ago

actually, if i got it right, the $ means for last occurrence of the number/string 12, right? cause it is so, it suppose to work.

Vera Gavriel Over a year ago

and when i use it in replace method the entire regex expression not suppose to be as a string , right? cause i have something like something.replace (/ + ToBeReplaced + $/, replaceValue)

Ro Yo Mi Over a year ago

I like the simplicity of this solution, and it does satisfy the sample text however this assumes the string 12 will be at the end of the string, which is different then the last occurrence of the 12.

redV Over a year ago

@anubhava Ok I understood. It was confusing because you took two examples each one with different purpose. +1 from me.

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