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I want to replace only a part of the string of a regex pattern match. I found this answer but I don't get it...
How do I use substitution?

Example of what I want: keep the first slug digit, only replace others

/09/small_image/09x/ > /09/thumbnail/
1st: unknown digit
2nd: "small_image"
3rd: unknown digit + "x"

Here is what I have so far:

var regexPattern = /\/\d\/small\_image\/\d*x/;
var regexPattern = /\/\d\/(small\_image\/\d*x)$1/;  ??

var result = regexPattern.test(str);
if (result) {
  str = str.replace(regexPattern, 'thumbnail');
}

I want to replace only a part of the string of a regex pattern match. I found this answer but I don't get it...
How do I use substitution?

Example of what I want: keep the first slug digit, only replace others

/09/small_image/09x/ > /09/thumbnail/
1st: unknown digit
2nd: "small_image"
3rd: unknown digit + "x"

Here is what I have so far:

var regexPattern = /\/\d\/small\_image\/\d*x/;
var regexPattern = /\/\d\/(small\_image\/\d*x)$1/;  ??

var result = regexPattern.test(str);
if (result) {
  str = str.replace(regexPattern, 'thumbnail');
}

• javascript
• regex

Share Improve this question Follow edited May 23, 2017 at 11:48 CommunityBot 111 silver badge asked Oct 10, 2016 at 3:07 user1775888user1775888 3,3031414 gold badges4949 silver badges6767 bronze badges 4

• Can you clarify what exactly you're trying to do, using complete sentences? I apologize if English isn't your first language – qxz Commented Oct 10, 2016 at 3:11
• thanks for reply, sorry english isnt my first language. I want to know how to only replace part of string not replace whole regex pattern match string. – user1775888 Commented Oct 10, 2016 at 3:13
• So the idea is that the first 09 and the 09 in 09x must match? – user663031 Commented Oct 10, 2016 at 3:38
• (1) What do you think $1 inside a regexp would mean? (Hint: it would mean the end of the string, followed by a 1.) (2) Why are you testing with RegExp.test before replacing? (Hint: you don't need to, since the replacement would not happen anyway unless there was a match.) (3) Why are you escaping the underscore? (Hint: you don't need to.) – user663031 Commented Oct 10, 2016 at 3:54

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2 Answers 2

Sorted by: Reset to default 19

var input = "/09/small_image/09x/";
var output = input.replace(/(\/\d+\/)small_image\/\d*x/, "$1thumbnail");
console.log(output);

Explanation:

Put the part you want to keep in parentheses, then refer to that as $1 in the replacement string - don't put $1 in your regex. So (\/\d+\/) means to match a forward slash followed by one or more digits, followed by another forward slash.

(Note that you don't need to escape underscores in a regex.)

Go with

var regexPattern = /(\/\d+\/)small\_image\/\d*x/;

and

str = str.replace(regexPattern, '$1thumbnail');

First, you were missing the +. Because 09 are two digits, you need the regexp to match one or more digits (\ḑ would be exactly one). This is accomplished by \d+
Second, everything you match is being removed at first. To get the /09/ part back afterwards, you have to remember it by putting it into brackets in the regexp (...) and afterwards reference it in the replacement via $1
One could as well create other groups and reference them by $2,$3 ...

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