foo := a.o b.o c.o
bar := $(foo:.o=.c)

This input sets ‘bar’ to‘a.c b.c c.c’. See Setting variables. A substitution reference is actually an abbreviation for use of the patsubst expansion function (see Functions for string substitution and analysis). We provide substitution references as well as patsubst for compatibility with other implementations of make.

Another type of substitution reference lets you use the full power of the patsubst function. It has the same form ‘$(var:a=b)’ described above, except that now a must contain a single ‘%’ character. This case is equivalent to ‘$(patsubst a, b,$(var))’. See Functions for string substitution and analysis for a description of the patsubst function. Consider the following example.
 

foo := a.o b.o c.o
bar := $(foo:%.o=%.c)

This statement sets ‘bar’ to‘a.c b.c c.c’.