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'

The first character of the string, or the first matching character? — jcaron
– jcaron, Commented Jun 27, 2014 at 11:03

anubhava · Accepted Answer · 2014-06-27 11:48:08Z

5

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");

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

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

Code:

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

Demo 2

edited Jun 27, 2014 at 11:48

answered Jun 27, 2014 at 11:09

anubhava

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9 Comments

Ammar Over a year ago

It's working fine for most of cases except for spaces. Any suggestion?

anubhava Over a year ago

Give me a sample value for which it doesn't work and I will tweak it.

anubhava Over a year ago

' ABC ' should be replaced with ' --ABC --' that appears to be totally new requirement. Can you clarify it?

Ammar Over a year ago

It's not working for multiple spaces in a sequence, when space it to be replaced with some other character.

Ammar Over a year ago

Actually I added 3 spaces before and after 'ABC' in the mentioned example, but editor replaced all with one space.

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zx81 · Accepted Answer · 2014-06-27 11:15:44Z

4

Negative Lookbehind and Beginning of String Anchor

Use the regex (?<!^| )A like this:

String resultString = subjectString.replaceAll("(?<!^| )A", "B");

In the demo, inspect the substitutions at the bottom.

Explanation

(?<!^| ) asserts that what immediately precedes the position is neither the beginning of the string nor a space character
A matches A

Reference

edited Jun 27, 2014 at 11:15

answered Jun 27, 2014 at 11:04

zx81

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4 Comments

zx81 Over a year ago

FYI, added explanation and demo. :)

zx81 Over a year ago

I edited the regex, demo and explanation, Ammar, please have a look. :)

zx81 Over a year ago

@AvinashRaj Yes, that would have worked too. But I really like to treat lookbehinds as "a real person", you know what I mean? Give it some meat, like for this C# regex we had earlier.

zx81 Over a year ago

Ammar, if you want to have more rules, for instance it can happen after several spaces... You just keep changing the simple lookbehind, and adding ORs if needed. For instance, (?<!^|[ ]*) if you can have multiple spaces before the first A, and (?<!^|[ ]*|[@_]) if you also don't want to replace the first A after a @ or _... It's a very very flexible method, you can keep expanding it.

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