JavaScript regex for adding space between numbers and operator symbols in a string

Question

I have a string containing numbers and operator symbols for example like this: 99*8998+-999 I need to have a space between an operator and adjacent digit like this: 99 * 8998 +- 999

I tried JavaScript replaceAll with regex: /(\d+)([/*+-]+)(\d*)/g But this regex works fine until there are no non adjacent operator symbols in the string.

For example refer following working and non working conditions.

"998998+999".replaceAll(/(\d+)([/*+-]+)(\d*)/g, "$1 $2 $3")  
=> "998998 + 999" // Works as expected

"998998+-999".replaceAll(/(\d+)([/*+-]+)(\d*)/g, "$1 $2 $3") 
=> "998998 +- 999" // Works as expected

"99*8998+-999".replaceAll(/(\d+)([/*+-]+)(\d*)/g, "$1 $2 $3") 
=> "99 * 8998+-999" // Does not works as expected. Expected: "99 * 8998 +- 999"

The regex seems to return after first match without capturing next one even with the global flag.

What could be wrong?

You can change the last group in a positive lookahead `(\d+)([\/*+-]+)(?=\d)` https://regex101.com/r/v53RVq/1 — The fourth bird, Sep 21 '20 at 16:33
The trailing `(\d*)` doesn't make much sense, because that section will match regardless, may as well just leave it off? — CertainPerformance, Sep 21 '20 at 16:34
Thanks both of you, just curious why my initial regex does not capture multiple match? Or it was not getting matched at all further? — Abhinandan Khilari, Sep 21 '20 at 16:37
See https://stackoverflow.com/questions/54883163/removing-whitespace-between-consecutive-numbers/54883192#54883192 — Wiktor Stribiżew, Sep 21 '20 at 16:37

anubhava · Answer 1 · 2020-09-21T16:41:50.963

Reason why your approach didn't work because your regex is matching and consuming characters in expression such as: 12+34-1 which has a overlapping match. Due to that you will have to use zero-width lookahead which will do only lookahead but won't consume characters from input.

You may match using this regex:

/(\d)(\+-|[\/*+-])(?=\d)/g

Replace using "$1 $2 "

RegEx Demo

There might be other 2 character operators for your case. I have used only +- in my example above. You can add then in alternations in 2nd capture group above.

RegEx Details:

(\d): Match and capture a digit in group #1
(\+-|[\/*+-]): Match +- or a single character operator /, *, + or -
(?=\d): Positive lookahead to assert that there is a digit ahead

JavaScript regex for adding space between numbers and operator symbols in a string

1 Answers1