a..dwould find all four-letter words beginning with 'A' and ending with 'D'. The search is case insensitive, so you may use upper or lower case letters.
Multiple letters may be specified for any position in the word. These are enclosed in square brackets:
A..[cfaj]would match all four-letter words beginning with 'A' and ending with 'c', 'f', 'a', or 'j'.
The multiple letters can also be expressed as a range:
[a-d]..rmatches all four-letter words beginning with 'a', 'b', 'c', or 'd' and ending with 'r'.
Multiple lists or ranges can be combined:
d..[a-dgjs-u]matches four-letter words beginning with 'd' and ending with 'a', 'b', 'c', 'd', 'g', 'j', 's', 't', or 'u'.
See also Multiple Characters and Ranges.
The software that drives the WordFinder is a collection of Unix shell scripts. Most of these scripts are available in my book, Shell Scripting Recipes: A Problem-Solution Approach (Apress, May 2005). You can run these scripts on any version of Unix, including Linux, Solaris, and the various BSD implementations. They have not been tested on Mac OS/X, but they should work there, too. (If you try them, please let me know how they run.)