sh-assignment-regexp

sh-assignment-regexp is a variable defined in `sh-script.el'.
Its value is

((csh . "\\<\\([[:alnum:]_]+\\)\\(\\[.+\\]\\)?[ 	]*[-+*/%^]?=")
 (ksh88 . "\\<\\([[:alnum:]_]+\\)\\(\\[.+\\]\\)?[ 	]*\\(?:[-+*/%&|~^]\\|<<\\|>>\\)?=")
 (bash . "\\<\\([[:alnum:]_]+\\)\\(\\[.+\\]\\)?\\+?=")
 (rc . "\\<\\([[:alnum:]_*]+\\)[ 	]*=")
 (sh . "\\<\\([[:alnum:]_]+\\)="))

Documentation:
Regexp for the variable name and what may follow in an assignment.
First grouping matches the variable name. This is upto and including the `='
sign. See `sh-feature'.

You can customize this variable.