Search & Replace - strange issue

[paulfiera]

I've been using the following regexps in Search and Replace

Search: <a class="([^"]*?)" id="([^"]*?)"></a>
Replace: 

Search: <a class="([^"]*?)"></a>
Replace:

Search: <a id="([^"]*?)"></a>
Replace:

But it looks like it doesn't always work doing and epub to epub conversion.

Just now, I ended with an epub with many

<a id="c1"></a>

Now, if I convert the epub again, the tags are gone, meaning it works the second time.

Maybe I'm doing something wrong?

Many thanks
Paul