Regular Expression to Replace All But One Character In String

Question

I need regular expression to replace all matching characters except the first one in a squence in string.

For example;

For matching with 'A' and replacing with 'B'

• 'AAA' should be replaced with 'ABB'

• 'AAA AAA' should be replaced with 'ABB ABB'

For matching with ' ' and replacing with 'X'

• '[space][space][space]A[space][space][space]' should be replaced with '[space]XXA[space]XX'

Answer 1

You need to use this regex for replacement:

\\BA

Working Demo

• \B (between word characters) assert position where \b (word boundary) doesn't match
• A matches the character A literally

Java Code:

String repl = input.replaceAll("\\BA", "B");

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UPDATE For second part of your question use this regex for replacement:

"(?<!^|\\w) "

Code:

String repl = input.replaceAll("(?<!^|\\w) ", "X");

Demo 2