Usually I use the notation:
L = letter
N = number
C = any character, letter, number
- = hyphen
With my described tool you can search for all kinds: LL, CCC, NNNN, L-L etc. It also will output lists of:
- soon expiring domains (potential drop candidates)
- domains in pendingDelete or redemptionPeriod (with higher chance to drop)
- free available domains
In combination with the "watch_domains" command one can let the tool watch for the pendindDelete/redemption candidates and alert you if one will get free. On Mac/Linux you might get an email alert on your smartphone (on Windows you must configure some command line e-mail program then it will work too).