Regex replace - Conditionally adding characters

Question

I am trying to do a regex find and replace, but in the replacement, I want to add a character only if a match was found for one of the matching groups. For example:

For the text ab I am using the regex (a)?(b)? to match multiple groups. I want the result to be replaced with \\1.\\2 to produce ab if and only if b is actually found. In the case that only a is found, the result should contain the period, ie just a . 应该包含的时期，也就是，只有a 。

I know this can be done in two steps, but is this kind of conditional insert possible with a single regex statement?

I oversimplified my test case a bit. My real situation is as follows:

I need to match name="name" property="property" in any order (the name field can come first or last.

I then want to replace it with value="name.property" or simply value="name" in the case property is not found.

Answer 1

Do it in multiple steps. First do the replacements that find both strings:

s/name="(.*?)" property="(.*?)"/value="\1.\2"/
s/property="(.*?)" name="(.*?)"/value="\2.\1"/

Then replace the ones that are left with just the one string:

s/name="(.*)"/value="\1"/

Answer 2

As said in several comments, there is no way to do this in a basic text editor's search and replace. It can only be done with callbacks or external code.

I ended up just doing it in two steps: Replace with \\1.\\2 , and then search and replace any dangling periods.

Answer 3

Yes it is possible:

Online Demo

([a])?[b](?(1)[c]|[d])

http://www.regular-expressions.info/conditional.html