Java regex in String replaceAll: negative look ahead does not work as expected

Question

I am trying to process text and replace all the occurence that start with "www." with a certain word (in this cases "Banana"), but I want to exclude all the cases in which there is "http://" before the "www".

When I use a positive lookahead it works (only the http://www case changes), but when I use a negative lookahead - both words change.

Can you help me with this one?

String baseString = "replace this: www.q.com and not this:http://www.q.com";
String positiveLookahead = baseString.replaceAll("(?:http://)www([^ \n\t]*)", "Banana");
String negativeLookahead = baseString.replaceAll("(?!http://)www([^ \n\t]*)", "Banana");

//positiveLookahead works (changes only the scond phrase), but positiveLookahead does not (changes both)

Shouldn't you be looking behind, not ahead?

khelwood
– khelwood

2018-08-23 10:55:18 +00:00
Commented Aug 23, 2018 at 10:55 — khelwood
– khelwood, Commented Aug 23, 2018 at 10:55
@khelwood You are right, see Wiktor response. Thanks!

Tal Sheffer
– Tal Sheffer

2018-08-23 11:17:55 +00:00
Commented Aug 23, 2018 at 11:17 — Tal Sheffer
– Tal Sheffer, Commented Aug 23, 2018 at 11:17

Wiktor Stribiżew · Accepted Answer · 2018-08-23 10:55:10Z

2

Use a negative lookbehind, (?<!http://):

String negativeLookahead = baseString.replaceAll("(?<!http://)www[^ \n\t]*", "Banana");

The (?<!http://)www[^ \n\t]* pattern matches:

(?<!http://) - a location in the string that is not immediately preceded with http://
www - a literal www substring
[^ \n\t]* - any 0+ chars other than space, newline and carriage return (probably you want to try \\S* instead).

answered Aug 23, 2018 at 10:55

Wiktor Stribiżew

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5 Comments

Tal Sheffer Over a year ago

Thanks, that's working :) So what does lookahead do?

Wiktor Stribiżew Over a year ago

@TalSheffer A negative lookahead matches a location in the string that is not immediately followed with a pattern, so (?!a)b is always equal to b as b is not an a.

Tal Sheffer Over a year ago

Got it. So what would be a legit use for a negative lookahead?

Wiktor Stribiżew Over a year ago

@TalSheffer Plenty. For example, see this today's answer of mine. More on negative lookahead here, and here.

Tal Sheffer Over a year ago

Thank you very much! Much appriciated!

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