Expression language to handle a capture group

[James McMahon]

I have an incoming filename that includes the pattern
prefix20221004035958postfix. I need to convert that yMdHms to seconds since
the epic within that filename.

This is the expression I attempt to use, but it seems to choke on the
capture group reference:
filename         ${filename:replaceFirst('([0-9]{14})', '*$1*
:toDate("yyyyMMddHHmmss","UTC"):toNumber()'}

How do I get this to work, applying expression language functions to the
capture group?
Thanks.