Hua-Ying,

You have to surround the match expression you want to use in your search
string with curly braces (called a "Tag expression"). Then these tag
expressions are referenced in the replace expression by index.  "\1" is the
first tag expression, "\2" is the second, and so on.

For example:

find: {.+} eats {.+}
replace: \2 is eaten by \1

       cat eats mouse
   ...becomes...
       mouse is eaten by cat

so, for your example

find: ^m{.+}
replace: k\1

Note that you don't need the "^" character (match beginning of line) in the
replace expression

Hope that helps.

-Scott