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I've used regular expressions in the past, but I can't figure something out that should not be so hard...

I have an app that parses a .csv file and replaces certain characters (commas). Here's an example..

Code:
item1,item2,item3,fruits are apples, oranges, grapes.  Squash is a vegetable, not a fruit.
What my app does is search each line between character1 and character18 and replace all "," with "~".

Does anyone know how this is done through regular expressions? I'm brushing up on it, but thought I'd post here in hopes of saving a little time.

TIA!