sudo gem install json

Building native extensions.  This could take a while...
ERROR:  Error installing json:
    ERROR: Failed to build gem native extension.

        /usr/local/bin/ruby extconf.rb
checking for ruby/re.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

Provided configuration options:
    --with-opt-dir
    --without-opt-dir
    --with-opt-include
    --without-opt-include=${opt-dir}/include
    --with-opt-lib
    --without-opt-lib=${opt-dir}/lib
    --with-make-prog
    --without-make-prog
    --srcdir=.
    --curdir
    --ruby=/usr/local/bin/ruby
/usr/local/lib/ruby/1.9.1/mkmf.rb:368:in `try_do': The complier failed to generate an executable file. (RuntimeError)
You have to install development tools first.
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:452:in `try_cpp'
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:834:in `block in have_header'
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:693:in `block in checking_for'
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:280:in `block (2 levels) in postpone'
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:254:in `open'
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:280:in `block in postpone'
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:254:in `open'
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:276:in `postpone'
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:692:in `checking_for'
    from /usr/local/lib/ruby/1.9.1/mkmf.rb:833:in `have_header'
    from extconf.rb:17:in `'


Gem files will remain installed in /usr/local/lib/ruby/gems/1.9.1/gems/json-1.5.1 for inspection.
Results logged to /usr/local/lib/ruby/gems/1.9.1/gems/json-1.5.1/ext/json/ext/generator/gem_make.out

> xcode-select --install