Need to detect length of text and fire script if it's 16, 20, or 36 chars long

Hi,

I'm a Javascript newbie and am scanning in two barcodes at a time into a PDF form. I need to check that the scanned code is either 16 chars (product code), 20 chars (batch Code), or 36 chars (valid combination - process it).

I'm trying to do this through the Format keystroke action, but an having trouble because the text field isn't updating when I thought it would. I'm open to any other method do do this.

Any ideas?

Thanks

James