How to replace a character in C# string ignoring other characters?

You can rather use HttpUtility.HtmlEncode & HttpUtility.HtmlDecode like below.

First decode your string to get normal string and then encode it again which will give you expected string.

HttpUtility.HtmlEncode(HttpUtility.HtmlDecode("hello a & b, <hello world >"));

• HttpUtility.HtmlDecode("hello a & b, <hello world >") will return hello a & b, <hello world >.

• HttpUtility.HtmlEncode("hello a & b, <hello world >") will return hello a & b, <hello world >

You could use regex, I suppose:

Regex.Replace("hello a & b, <hello world >", "&(?![a-z]{1,};)", "&");

• & match literal &
• (?! ) negative lookahead (assert that the following does not match)
• [a-z]{1,}; any char a-z, one or more times, followed by a single ';'