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→INVESTIGATION 1: SIMPLE & COMPLEX REGULAR EXPRESSIONS
# Issue the '''ls''' Linux command to confirm that the text file was downloaded.<br><br>
# View the contents of the '''textfile1.txt''' file using the '''more''' command and quickly view the contents of this file.<br><br>Although there are several Linux commands that use regular expressions,<br>we will only be using the '''grep''' command for this investigation.<br><br>
#Issue the following Linux pipeline command to match the pattern "'''the'''" within '''textfile1.txt''':<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "the" textfile1.txt</span><br><br>Take a few moments to view the output and observe the matched pattern.<br><br># Now, issue the grep Linux pipeline command with the <span style="font-weight:bold;font-family:courier;">-i</span> option to ignore case sensitively:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep -i "the" textfile1.txt</span><br><br>What do you notice is different when issuing this pipeline command?<br><br>You will notice that the pattern "'''the'''" is matched including larger words like "'''them'''" and "'''their'''".<br>You can issue the '''grep''' command with the -w option to only match the pattern as a '''word'''.<br><br># Issue the following Linux pipeline command:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep -w -i "the" textfile1.txt</span><br><br>You should now see only strings of text that match the word '''"the"''' (upper or lower case).<br><br>Matching literal or simple regular expressions can be useful, but are '''limited'''<br>in what they can assist with pattern matching. For example, you may want to<br>search for a pattern located at the '''beginning''' or '''end''' of the string.<br><br>There are other regular expression symbols that provide more '''precise''' pattern matches.<br>These special characters are known as '''complex''' and '''extended''' regular expressions symbols.<br> In this section, we will focus on complex ''regular expressions'' and then discuss<br>''extended regular expressions'' in INVESTIGATION 2.<br><br># Issue the following Linux pipeline command:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep -w -i "^the" textfile1.txt</span><br><br>The '''^''' symbol is referred to as an '''anchor'''. In this case, it only matches<br>the word "'''the'''" (both upper or lowercase) at the <u>beginning</u> of the string.<br><br># Issue the following Linux pipeline command:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep -w -i "the$" textfile1.txt</span><br><br>The '''$''' symbol is used to anchor patterns at the <u>end</u> of the string.<br><br># Issue the following Linux pipeline command to anchor the <u>word</u> "'''the'''"<br>simultaneously at the <u>beginning</u> and <u>end</u> of the string:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep -w -i "^the$" textfile1.txt </span><br><br>What do you notice?<br><br>Anchoring patterns at both the <u>beginning</u> and <u>ending</u> of strings can greatly assist<br>for more '''precise''' search patterns.<br><br>We will now be demonstrate the power of anchoring<br>combined with other complex regular expressions symbols.<br><br>
# Issue the following Linux command to match strings that begin with 3 characters:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^..." textfile1.txt</span><br><br>What do you notice? Can lines that contain '''less than 3 characters''' be displayed?<br><br>
# Issue the following Linux command to match strings that begin and end with 3 characters:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^...$" textfile1.txt</span><br><br>What do you notice compared to the previous command?<br><br>
# Let's issue a Linux 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 Linux 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</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 run a '''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 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 pipeline command to display only whole numbers:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[0-9]*$" numbers1.dat | tee faulty.txt</span><br><br>You may have noticed that the command does not entirely work. You may notice an empty line<br>(which is NOT a whole number). This occurs since the * regular expression symbol represents<br>ZERO or MORE occurrences of a number. You can use an additional numeric character class<br>with the * regular expression symbol to search for one or more occurrences of a number.<br><br># Issue the following linux Linux pipeline command to display only whole numbers:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[0-9][0-9]*$" numbers1.dat | tee whole.txt</span><br><br>You should see that this now works.<br><br># Issue the following linux Linux pipeline command to display whole positive or negative integers:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[+-][0-9][0-9]*$" numbers1.dat | tee signed.txt</span><br><br>What did you notice?<br><br># Issue the following linux Linux pipeline command to display only whole numbers (with or without a positive or negative sign):<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[+-]*[0-9]*$" numbers1.dat | tee all.txt</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>
:Proceed to INVESTIGATION 2.
