Macro Characters
Macro characters used by various libraries:

direct character definition

Char Meaning Used by
λ (lambda) LAMBDA lambda-reader
Λ (Lambda) LAMBDA lambda-reader

# dispatch

(Note that the subchar dispatching is done case insensitively, so we list only uppercase letters, but the corresponding lower case letter may be used too (and might be used often for better readability, notably when a numerical argument is present before and after the subchar)).

.
Char Meaning Used by
I Infix syntax Infix
? String interpolation cl-interpol
` (backquote) Shell command output substitution regex-clawk-lexer
U URLs CL-HTTP Puri uri-template cl-uri-templates
_ (underscore) FFI Traps MCL
_ (underscore) A lookup for Qt functionality CommonQt
$ FFI Constants MCL
! Unix shell scripting (known as "shebang") CLISP and others
@ read-time splicing cl-rttemplate
" (double quote) a read-time quasiquote cl-rttemplate
" (double quote) pathnames (predates #P?) CLISP
L Lambda: #L(list* !1 '- !2) as shorthand for two-arg LAMBDA iterate
M Money: #978m123.43 = 123.43 EUR ; #840m123.43 = 123.43 USD in com.informatimago.common-lisp.invoice (should be extracted as a stand-alone library, bug Pascal Bourguignon