Using regex to replace only the last occurrence of a pattern with JS

Question

I have a case where I'm trying to replace a certain pattern with another. My problem is that I need to only replace the last occurrence of that pattern, not all of them. I've found this question:

How to replace last occurrence of characters in a string using javascript

But it doesn't fit my needs. As a background, I will say that I am trying to replace a CSS rule, but for the current example lets look at this text:

abcd:bka:

bbb:aad:

accx:aaa:

bbb:a0d:

cczc:aaa:

lets say I only want to replace the value of bbb. My current rule will be

text.replace(/(\s*bbb:)([^:]+)/,"$1aaa")

but it will only replace the first match, while I want it to replace the last one. My current pattern is actually more complex than this one, but I think the pseudo problem will suffice.

Answer 1

Try

text.replace(/(\s*bbb:)(?![\s\S]*bbb:)[^:]+/,"$1aaa")

The negative lookahead assertion makes sure that there is no further bbb: ahead in the text. The parentheses around [^:]+ are unnecessary.

Explanation:

(?!       # Assert that it is impossible to match the following after the current position:
 [\s\S]*  # any number of characters including newlines
 bbb:     # the literal text bbb:
)         # End of lookahead assertion

The [\\s\\S] workaround is necessary because JavaScript doesn't have an option to allow the dot to match newlines.