CODEWITHSUNDEEP
javaregex
user2045279
user2045279

Reputation: 711

Using Regular Expressions to replace a select instance of a character

First and foremost, this is not a homework/school question. This is something I needed for a personal project and was able to generalize the problem using simple characters as follows:

Create a regular expression which can be used in the java string replaceAll function for an input string aaabaabaa (or similar) and converts each a to A, UNLESS it is preceded by a b. So the expected output is AAAbaAbaA. I've been trying this for at least 2 hours now... The best I could come up with was:

replaceAll("^a|([^b])a", "$1A");

This fails on aaa, where the return value is AaA

I'm trying to say "Any 'a' without a 'b' before it should be an A. Any ideas for a RegEx novice? Thanks very much!

Upvotes: 0

Views: 67

Answers (1)

p.s.w.g
p.s.w.g

Reputation: 149050

Try using a negative lookbehind:

str.replaceAll("(?<!b)a", "A");

This will match any a not immediately preceded by a b.

Upvotes: 3

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