From a tag like this:

<a name="Introduction"></a><h1><span class="mw-headline" >Introduction</span></h1>

I'd like to make:

<a name="Introduction"></a><h1><span class="mw-headline" id="Introduction" >Introduction</span></h1>

Therefore I do the following replacement:

Match:
<a name="([A-Za-z0-9 ?_.]+)"></a><h([123])>[^mw]*mw-headline" >([A-Za-z0-9 ?_.]+)</span></h[123]>

And replace it with:

<a name="\1"></a><h\2><span class="mw-headline" id="\1" >\3</span></h\2>

This works when using a find and replace editor which accepts regex. But I can't seem to fit it in one sed command.