TextMate Snippet to Mirror Only Matched Characters

I have been trying unsucces开发者_StackOverflowsfully to mirror text that only matches "a-z0-9" using the following code:

${1/[a-z]/$0/}${1}

After the snippet has been tab triggered I would expect to type "$test" and see "test" mirrored.

Any clues? Many thanks!

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The snippet beneath removes all $, % and ? from either beginning or end of your text. Note that the regular expression is added and applied to the mirror, not to the input itself.

$1 ${1/[\$\?%]*([^\$\?%]*)[\$\?%]*/$1/} $0

If you want to exclude more characters, add those to each class (escape them if necessary).