Been trying to install adhearsion recently and encountered the following error:

ERROR: Error installing adhearsion:
ERROR: Failed to build gem native extension.

/Users/mynamehere/.rvm/rubies/ruby-1.9.3-p327/bin/ruby extconf.rb
checking for pcre.h... no
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PCRE is missing. You must install it as per the README @ https://github.com/benlangfeld/ruby_speech
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*** 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.

It turns out you just need to go to ruby_speech plugin and follow the README and execute the following command:

brew install pcre