[TxMt] mirroring transformed tab-stops

[Oliver Taylor]

I'm using the standard heading snippet to automatically add an ID to  
a heading, like this:

<h3 id="${1/[[:alpha:]]+|( )/(?1:_:\L$0)/g}">${1:Some Title}</h3>

But now I want to mirror the transformed ID value in a permalink  
later in the snippet like this:

<a href="http://somesite/links/#$1>permalink</a>

If I mirror $1 I get the untransformed text.

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Is there a way to mirror transformed tab-stops?