How can I extract dates from a funky filename (date's in name as yyyyMMdd)

here's example, but, last line doesn't convert [datetime].

$pattern = "(?<text>.*)(?<date>\d{8})(?<text2>.*)"  "a_12345678_at.txt" -match $pattern | out-null  ([datetime] ($matches.date)).tostring('mm-dd-yyyy')

throws error:

cannot convert value "20120224" type "system.datetime". error: "string not recognized valid datetime."  @ line:1 char:23  + ([datetime] ($matches. <<<< date)).tostring('mm-dd-yyyy')      + categoryinfo          : notspecified: (:) [], runtimeexception      + fullyqualifiederrorid : runtimeexception

see mjolinor, using regexes... :)  i threw in group names.

ps >  [datetime]::parseexact("20120224","yyyymmdd",$null).tostring('mm-dd-yyyy') 02-24-2012

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