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.

Tim Pietzcker · Accepted Answer · 2011-01-20 08:02:47Z

8

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.

answered Jan 20, 2011 at 8:02

Tim Pietzcker

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2 Comments

Peter Ehrlich Over a year ago

I like this answer: stackoverflow.com/questions/843049/… Preface with (.*) to eat up as much as possible before matching.

Tim Pietzcker Over a year ago

@PeterEhrlich: You'd have to use [\s\S]* instead (the dot doesn't match newlines), and I generally prefer being explicit to using unspecific regexes like .*, but yes, that would work, too. Whether its performance will be better or worse than mine depends on the actual text this is being used on. I'd suspect that it's going to be worse on large texts because of all the backtracking that .* causes.

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