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→INVESTIGATION 1: SIMPLE & COMPLEX REGULAR EXPRESSIONS
# Let's issue a command to display strings that contain more than one occurrence of the letter "x":<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "xx*" textfile1.txt</span><br><br>Why did this work? because the pattern indicates one occurrence of the letter "x",<br>followed by zero or MORE occurrences of the letter "x".<br><br>If you combine the complex regular expression symbols ".*" it will act like<br>zero or more occurrences of any character (i.e. like "*" did in filename expansion).<br><br>
# Issue the following command to match strings begin and end with a number with nothing or anything inbetween:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[0-9].*[0-9]$" textfile1.txt</span><br><br>Using '''simultaneous anchors''' combined with the ".*" symbol(s) can help you to refine your search patterns of strings.<br><br>
# Issue the following linux pipeline command to display strings that begin with a capital letter,<br>end with a number, and contains a capital X somewhere inbetween:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[A-Z].*X.*[0-9]$" textfile1.txt | more</span><br><br>Let's look at another series of examples involving searching for strings that only contain '''valid numbers'''.<br>We will use pipeline commands to both display stdout to the screen and save to files<br>for confirmation of running these pipeline commands when running checking-script later in this investigation.<br><br>
# Issue the following Linux command to create the '''regexps''' directory: <span style="color:blue;font-weight:bold;font-family:courier;">mkdir ~/regexps</span><br><br>
# Change to the '''regexps''' directory and confirm that you have moved to this directory.<br><br>
# First, issue the following linux command to download another data file called '''numbers1.dat''':<br><span style="color:blue;font-weight:bold;font-family:courier;">wget <nowiki>https://ict.senecacollege.ca/~murray.saul/uli101/numbers1.dat</nowiki></span><br><br>
# View the contents of the '''numbers.dat''' file using the '''more''' command and quickly view the contents of this file.<br>You should notice valid and invalid numbers contained in this file. When finished, exit the more command.<br><br>
# Issue the following linux command to display only whole numbers:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[0-9]*$" numbers1.dat | tee faulty.txt | more</span><br><br>You may have noticed that the command does not entirely work. You may notice an empty line<br>(which is NOT a whole number). This occurs since the * regular expression symbol represents<br>ZERO or MORE occurrences of a number. You can use an additional numeric character class<br>with the * regular expression symbol to search for one or more occurrences of a number.<br><br># Issue the following linux command to display only whole numbers:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[0-9][0-9]*$" numbers1.dat | tee whole.txt | more</span><br><br>You should see that this works.<br><br># Issue the following linux command to display whole positive or negative integers:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[+-][0-9][0-9]*$" numbers1.dat | tee signed.txt | more</span><br><br>What did you notice?<br><br># Issue the following linux command to display only whole numbers (with or without a positive or negative sign):<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[+-]*[0-9]*$" numbers1.dat | tee all.txt | more</span><br><br>Did this command work?<br><br>
# Issue the following Linux command to check that you created those hard links: <br><span style="color:blue;font-weight:bold;font-family:courier;">bash /home/murray.saul/scripts/week9-check-1</span><br><br>If you encounter errors, then view the feedback to make corrections, and then re-run the checking script.<br>If you receive a congratulation message that there are no errors, then proceed with this tutorial.<br><br>
