Useful links to learn Ruby/Python/RegularExpressions/SQL

Following links might be helpful for you to learn Ruby/Python/Regular Expressions/SQL. Even C is also available.

Ruby: http://ruby.learncodethehardway.org/book/  
Python: http://learnpythonthehardway.org/book/
Regular Expressions: http://regex.learncodethehardway.org/book/
SQL: http://sql.learncodethehardway.org/book/
C: http://c.learncodethehardway.org/book/