Regular expression to link twitter users…

Replace this '@(\w+)'
With this '<a href="http://twitter.com/\1">@\1</a>'

You can use locate to find files with name that matches a said regexp…

locate -i -r '\.jpg$'

Print the contents of a file from a given regular expression to another

sed -n '/start/,/end/ p' file

This will print the contents of the ‘file’ from the line that matches /start/ until the line that matches /end/

Print a file until a regular expression is matched.

cat file.txt | perl -pe "exit if(/Thats all/)"