Get a string from description

Get a string from description

Hi dears,
How I can get a value from description and set it on a custom filed?
for ex: a request registered, on the description has like :
 from [Joe Evian Jayas Pen [HTML invoic](/moyaja/pen/wJCra/)....
 Invoice #: 101138
Amount Due$600.00
How I can get  101138 or  $600.00 on the description and set it on a custom filed?
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