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8.2 Functions for String Substitution and Analysis
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Here are some functions that operate on strings:

'$(subst FROM,TO,TEXT)'
     Performs a textual replacement on the text TEXT: each occurrence of
     FROM is replaced by TO.  The result is substituted for the function
     call.  For example,

          $(subst ee,EE,feet on the street)

     produces the value 'fEEt on the strEEt'.

'$(patsubst PATTERN,REPLACEMENT,TEXT)'
     Finds whitespace-separated words in TEXT that match PATTERN and
     replaces them with REPLACEMENT.  Here PATTERN may contain a '%'
     which acts as a wildcard, matching any number of any characters
     within a word.  If REPLACEMENT also contains a '%', the '%' is
     replaced by the text that matched the '%' in PATTERN.  Words that
     do not match the pattern are kept without change in the output.
     Only the first '%' in the PATTERN and REPLACEMENT is treated this
     way; any subsequent '%' is unchanged.

     '%' characters in 'patsubst' function invocations can be quoted
     with preceding backslashes ('\').  Backslashes that would otherwise
     quote '%' characters can be quoted with more backslashes.
     Backslashes that quote '%' characters or other backslashes are
     removed from the pattern before it is compared file names or has a
     stem substituted into it.  Backslashes that are not in danger of
     quoting '%' characters go unmolested.  For example, the pattern
     'the\%weird\\%pattern\\' has 'the%weird\' preceding the operative
     '%' character, and 'pattern\\' following it.  The final two
     backslashes are left alone because they cannot affect any '%'
     character.

     Whitespace between words is folded into single space characters;
     leading and trailing whitespace is discarded.

     For example,

          $(patsubst %.c,%.o,x.c.c bar.c)

     produces the value 'x.c.o bar.o'.

     Substitution references (*note Substitution References:
     Substitution Refs.) are a simpler way to get the effect of the
     'patsubst' function:

          $(VAR:PATTERN=REPLACEMENT)

     is equivalent to

          $(patsubst PATTERN,REPLACEMENT,$(VAR))

     The second shorthand simplifies one of the most common uses of
     'patsubst': replacing the suffix at the end of file names.

          $(VAR:SUFFIX=REPLACEMENT)

     is equivalent to

          $(patsubst %SUFFIX,%REPLACEMENT,$(VAR))

     For example, you might have a list of object files:

          objects = foo.o bar.o baz.o

     To get the list of corresponding source files, you could simply
     write:

          $(objects:.o=.c)

     instead of using the general form:

          $(patsubst %.o,%.c,$(objects))

'$(strip STRING)'
     Removes leading and trailing whitespace from STRING and replaces
     each internal sequence of one or more whitespace characters with a
     single space.  Thus, '$(strip a b c )' results in 'a b c'.

     The function 'strip' can be very useful when used in conjunction
     with conditionals.  When comparing something with the empty string
     '' using 'ifeq' or 'ifneq', you usually want a string of just
     whitespace to match the empty string (*note Conditionals::).

     Thus, the following may fail to have the desired results:

          .PHONY: all
          ifneq   "$(needs_made)" ""
          all: $(needs_made)
          else
          all:;@echo 'Nothing to make!'
          endif

     Replacing the variable reference '$(needs_made)' with the function
     call '$(strip $(needs_made))' in the 'ifneq' directive would make
     it more robust.

'$(findstring FIND,IN)'
     Searches IN for an occurrence of FIND.  If it occurs, the value is
     FIND; otherwise, the value is empty.  You can use this function in
     a conditional to test for the presence of a specific substring in a
     given string.  Thus, the two examples,

          $(findstring a,a b c)
          $(findstring a,b c)

     produce the values 'a' and '' (the empty string), respectively.
     *Note Testing Flags::, for a practical application of 'findstring'.

'$(filter PATTERN...,TEXT)'
     Returns all whitespace-separated words in TEXT that _do_ match any
     of the PATTERN words, removing any words that _do not_ match.  The
     patterns are written using '%', just like the patterns used in the
     'patsubst' function above.

     The 'filter' function can be used to separate out different types
     of strings (such as file names) in a variable.  For example:

          sources := foo.c bar.c baz.s ugh.h
          foo: $(sources)
                  cc $(filter %.c %.s,$(sources)) -o foo

     says that 'foo' depends of 'foo.c', 'bar.c', 'baz.s' and 'ugh.h'
     but only 'foo.c', 'bar.c' and 'baz.s' should be specified in the
     command to the compiler.

'$(filter-out PATTERN...,TEXT)'
     Returns all whitespace-separated words in TEXT that _do not_ match
     any of the PATTERN words, removing the words that _do_ match one or
     more.  This is the exact opposite of the 'filter' function.

     For example, given:

          objects=main1.o foo.o main2.o bar.o
          mains=main1.o main2.o

     the following generates a list which contains all the object files
     not in 'mains':

          $(filter-out $(mains),$(objects))

'$(sort LIST)'
     Sorts the words of LIST in lexical order, removing duplicate words.
     The output is a list of words separated by single spaces.  Thus,

          $(sort foo bar lose)

     returns the value 'bar foo lose'.

     Incidentally, since 'sort' removes duplicate words, you can use it
     for this purpose even if you don't care about the sort order.

'$(word N,TEXT)'
     Returns the Nth word of TEXT.  The legitimate values of N start
     from 1.  If N is bigger than the number of words in TEXT, the value
     is empty.  For example,

          $(word 2, foo bar baz)

     returns 'bar'.

'$(wordlist S,E,TEXT)'
     Returns the list of words in TEXT starting with word S and ending
     with word E (inclusive).  The legitimate values of S start from 1;
     E may start from 0.  If S is bigger than the number of words in
     TEXT, the value is empty.  If E is bigger than the number of words
     in TEXT, words up to the end of TEXT are returned.  If S is greater
     than E, nothing is returned.  For example,

          $(wordlist 2, 3, foo bar baz)

     returns 'bar baz'.

'$(words TEXT)'
     Returns the number of words in TEXT.  Thus, the last word of TEXT
     is '$(word $(words TEXT),TEXT)'.

'$(firstword NAMES...)'
     The argument NAMES is regarded as a series of names, separated by
     whitespace.  The value is the first name in the series.  The rest
     of the names are ignored.

     For example,

          $(firstword foo bar)

     produces the result 'foo'.  Although '$(firstword TEXT)' is the
     same as '$(word 1,TEXT)', the 'firstword' function is retained for
     its simplicity.

'$(lastword NAMES...)'
     The argument NAMES is regarded as a series of names, separated by
     whitespace.  The value is the last name in the series.

     For example,

          $(lastword foo bar)

     produces the result 'bar'.  Although '$(lastword TEXT)' is the same
     as '$(word $(words TEXT),TEXT)', the 'lastword' function was added
     for its simplicity and better performance.

   Here is a realistic example of the use of 'subst' and 'patsubst'.
Suppose that a makefile uses the 'VPATH' variable to specify a list of
directories that 'make' should search for prerequisite files (*note
'VPATH' Search Path for All Prerequisites: General Search.).  This
example shows how to tell the C compiler to search for header files in
the same list of directories.

   The value of 'VPATH' is a list of directories separated by colons,
such as 'src:../headers'.  First, the 'subst' function is used to change
the colons to spaces:

     $(subst :, ,$(VPATH))

This produces 'src ../headers'.  Then 'patsubst' is used to turn each
directory name into a '-I' flag.  These can be added to the value of the
variable 'CFLAGS', which is passed automatically to the C compiler, like
this:

     override CFLAGS += $(patsubst %,-I%,$(subst :, ,$(VPATH)))

The effect is to append the text '-Isrc -I../headers' to the previously
given value of 'CFLAGS'.  The 'override' directive is used so that the
new value is assigned even if the previous value of 'CFLAGS' was
specified with a command argument (*note The 'override' Directive:
Override Directive.).