Regexp to recognize a character in an abbreviation or expansion.
This regexp will be surrounded with \\( ... \\) when actually used.
Set this variable to "\\sw" if you want ordinary words or
"\\sw\\|\\s_" if you want symbols (including characters whose
syntax is "symbol" as well as those whose syntax is "word".
The value nil has a special meaning: the abbreviation is from point to
previous word-start, but the search is for symbols.
For instance, if you are programming in Lisp, `yes-or-no-p' is a symbol,
while `yes', `or', `no' and `p' are considered words. If this
variable is nil, then expanding `yes-or-no-' looks for a symbol
starting with or containing `no-'. If you set this variable to
"\\sw\\|\\s_", that expansion looks for a symbol starting with
`yes-or-no-'. Finally, if you set this variable to "\\sw", then
expanding `yes-or-no-' signals an error because `-' is not part of a word;
but expanding `yes-or-no' looks for a word starting with `no'.
The recommended value is nil, which will make dabbrev default to using "\\sw\\|\\s_".