Using the matched expression in replacement
Here is an example of using the matched expression in replacement:
s/%(..)/pack("c",hex($1))/ge;
The left-hand side (%(..)) finds a % and puts the two characters that follow in a subexpression.
The right side uses that subexpression as the argument to the hex function, then converts the resulting decimal number to a character with pack.
Some example REs
As was mentioned earlier, it's probably best to build up your use of regular expressions slowly. Here are a few examples.
Remember that to use them for matching they should be put in /.../ slashes
[01] # Either "0" or "1"
\/0 # A division by zero: "/0"
\/ 0 # A division by zero with a space: "/ 0"
\/\s0 # A division by zero with a whitespace:
# "/ 0" where the space may be a tab etc.
\/ *0 # A division by zero with possibly some
# spaces: "/0" or "/ 0" or "/ 0" etc.
\/\s*0 # A division by zero with possibly some
# whitespace.
\/\s*0\.0* # As the previous one, but with decimal
# point and maybe some 0s after it. Accepts
# "/0." and "/0.0" and "/0.00" etc and
# "/ 0." and "/ 0.0" and "/ 0.00" etc.