The string entity consists of all the characters between the cursor and
 the next occurrence of a specified search string.  Specify the string
 entity by enclosing the desired search string in single or double
 quotation marks. If the string is null (specified by '' or ""), the
 previous search string is used again.

 For key definitions, you may use the null character (character code 0)
 in place of the quotation marks to avoid conflicts with the search
 string.