DataLinker allows you to use regular expressions (regex) to modify data before it's inserted into an InDesign document. You might have also seen these referred to as GREP expressions in InDesign
Where to find the settings
You can do this with alphanumeric and barcode fields. You set the regex find and replace string in the Field Options dialog (right click on any field in the DataLinker palette and choose "Edit field").
Here's where to find the options in the barcode field settings:
And here's where they are in the alphanumeric field settings:
How to use them
The find string is what you're looking for in each field, and the replace field is what you're replacing. Regexes are very powerful - here's a good starting spot. Here's Adobe's help page on using GREP - most of what's there applies here too.