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)

Answer 1

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).