Normally, dynamic abbrev expansion ignores case when searching for
expansions. That is, the expansion need not agree in case with the word
you are expanding. If you set dabbrev-case-fold-search to
nil, then the word and the expansion must match in case.
The value of dabbrev-case-fold-search may be any expression.
Dynamic abbrev expansion evaluates that expression, and ignores case
while searching if its value is not nil. The default value of
dabbrev-case-fold-search is case-fold-search, so normally
the value of case-fold-search controls the decision. The reason
why dynamic abbrev expansion normally ignores case when searching for
expansions is that normally the value of case-fold-search is
t.
Normally, dynamic abbrev expansion preserves the case pattern of
the word you are expanding, by converting the expansion to that case
pattern. If you set dabbrev-case-replace to nil, the
expansion is copied without conversion.
The variables dabbrev-case-fold-search and
dabbrev-case-replace are handled in a special way. Their values
are actually Lisp expressions which are evaluated each time a decision
needs to be made. If the expression's value is non-nil, then
case is ignored in searching, or converted on replacement, respectively.
If the expression's value is nil, case is not ignored or not
converted. The default values let the variables case-fold-search
(see section Searching and Case) and case-replace (see section Replace Commands and Case) control what to do.
The variable dabbrev-abbrev-char-regexp, if non-nil,
controls which characters are considered part of a word, for dynamic expansion
purposes. The regular expression must match just one character, never
two or more. The same regular expression also determines which
characters are part of an expansion. The value nil has a special
meaning: abbreviations are made of word characters, but expansions are
made of word and symbol characters.
In shell scripts and makefiles, a variable name is sometimes prefixed
with `$' and sometimes not. Major modes for this kind of text can
customize dynamic abbreviation to handle optional prefixes by setting
the variable dabbrev-abbrev-skip-leading-regexp. Its value
should be a regular expression that matches the optional prefix that
dynamic abbreviation should ignore.