How to replace line-breaks with commas using grep in TextWrangler?

1. Choose Find from the Search menu. TextWrangler opens the Find window.
2. Select the "Grep" checkbox
3. Type the string you are looking for ("\n" or "\r\n" or "\r") in the Find textfield.
4. Type the replace string (", ") in the Replace text field.
5. Click "Replace All"

See chapters 7 and 8 of the TextWrangler User Manual if you have problems.