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→INVESTIGATION 1: SIMPLE & COMPLEX REGULAR EXPRESSIONS
# Issue the following 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 command to match strings that begin and end with 3 characters:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^...$" textfile1.txt</span><br><br>What do you notice compared to the previous command?<br><br>
# Issue the following command to match strings that begin with 3 digits:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[0-9][0-9][0-9]" textfile1.txt</span><br><br>Did only What did you notice lines containing only 3 digits appear?<br><br># Issue the following command to match strings that end with 3 uppercase letters:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "[A-Z][A-Z][A-Z]$" textfile1.txt</span><br><br>Did any lines match this pattern?<br><br># Issue the following command to match strings that consist of only 3 digits:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[0-9][0-9][0-9]$" textfile1.txt</span><br><br>What did you notice?<br><br>The '''*''' complex regular expression symbol is often confused with filename expansion. <br>In other words, it does NOT represent zero or more of '''any character''', but zero or or occurrences of the character that comes before the * symbol.<br><br>
# To demonstration, issue the following command to display zero or more occurrences of the letter x:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "x*" textfile1.txt</span><br><br>You will most likely notice most lines of the file is displayed.<br><br>
# 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, followed by zero or MORE occurrences of the letter x.<br><br>If you combine the complex regular expression symbols .* it will act like zero or more occurrence of any character (like * did in filename expansion).<br><br>
