How do I display data from the beginning of a file until the first occurrence of a regular expression?

Question

How do I display data from the beginning of a file until the first occurrence of a regular expression?

For example, if I have a file that contains:

One
Two
Three
Bravo
Four 
Five

I want to start displaying the contents of the file starting at line 1 and stopping when I find the string "B*". So the output should look like this:

One
Two
Three

Answer 1

perl -pe 'last if /^B/' source.txt

An explanation: the -p switch adds a loop around the code, turning it into this:

while ( <> ) {
    last if /^B.*/;  # The bit we provide
    print;
}

The last keyword exits the surrounding loop immediately if the condition holds - in this case, /^B/, which indicates that the line begins with a B.