A substitution reference substitutes the value of a variable with alterations that you specify. It has the form `$(var:a=b)' (or `${var:a=b}') and its meaning is to take the value of the variable var, replace every a at the end of a word with b in that value, and substitute the resulting string.
When we say "at the end of a word", we mean that a must appear either followed by whitespace or at the end of the value in order to be replaced; other occurrences of a in the value are unaltered. For example:
foo := a.o b.o c.o bar := $(foo:.o=.c)
sets `bar' to `a.c b.c c.c'. See section Setting Variables.
A substitution reference is actually an abbreviation for use of the
patsubst expansion function (see section Functions for String Substitution and Analysis). We provide
substitution references as well as patsubst for compatibility with
other implementations of make.
Another type of substitution reference lets you use the full power of
the patsubst function. It has the same form
`$(var:a=b)' described above, except that now
a must contain a single `%' character. This case is
equivalent to `$(patsubst a,b,$(var))'.
See section Functions for String Substitution and Analysis,
for a description of the patsubst function.
For example: foo := a.o b.o c.o bar := $(foo:%.o=%.c)
sets `bar' to `a.c b.c c.c'.