How to do it...

Let's get started:

1. Open a terminal, and an editor of your choice to create a new script.
2. Inside of your script, add the following:

#!/bin/bash
STR1='123 is a number, ABC is alphabetic & aBC123 is alphanumeric.'

echo "-------------------------------------------------"
# Want to find all of the files beginning with an uppercase character and end with .pdf?
ls * | grep [[:upper:]]*.pdf

echo "-------------------------------------------------"
# Just all of the directories in your current directory?
ls -l [[:upper:]]*

echo "-------------------------------------------------"
# How about all of the files we created with an expansion using the { } brackets?
ls [:lower:].test .

echo "-------------------------------------------------"
# Files with a specific extension OR two?
echo ${STR1} > test.txt
ls *.{test,txt} 

echo "-------------------------------------------------"
# How about looking for specific punctuation and output on the same line
echo "${STR1}" | grep -o [[:punct:]] | xargs echo

echo "-------------------------------------------------"
# How about using groups and single character wildcards (only 5 results)
ls | grep -E "([[:upper:]])([[:digit:]])?.test?" | tail -n 5

exit 0

3. Now, execute the script and your console should be flooded with the output. Most importantly, let's look at the last five results. Notice the Z9(,) and Z9.test(3) among the results? This is the power of a regex at work! Okay, so we get that we can now create and search for a bunch of folders or files using variables, but can I use regexes to find things like variable parameters? Absolutely! See the next step.
4. In the console, try the following:

$ grep -oP 'name="\K.*?(?=")' www.packtpub.com/index.html

5. Again, in the console, try the following:

$ grep -P 'name=' www.packtpub.com/index.html

6. Can we do better using commands like tr to remove new lines when finding instances of IF that may span multiple lines?

$ tr '\n' ' ' < www.packtpub.com/index.html | grep -o '<title>.*</title>' 

7. Now, let's remove a bit more gunk from the screen using cut as a finale. Usually, the console is 80 characters wide, so let's add a line number and trim the output from grep:

$ grep -nP 'name=' www.packtpub.com/index.html | cut -c -80