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→INVESTIGATION 1: SIMPLE & COMPLEX REGULAR EXPRESSIONS
# Issue the following Linux 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]$" textfile1.txt</span><br><br>Did any lines match this pattern?<br><br>
# Issue the following Linux command to match strings that consist of only 3 digits:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[0-9][0-9][0-9]$" textfile1.txt</span><br><br>What did you notice?<br><br>
# Issue the following Linux command to match strings that consist of only 3 alphanumeric digits:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9]$" textfile1.txt</span><br><br>What did you notice?<br><br>The <span style="font-weight:bold;font-family:courier;">"*"</span> complex regular expression symbol is often confused with the filename expansionsymbol.<br>In other words, it does NOT represent zero or more of '''any character''', but zero or more occurrences<br>of the character that comes '''before''' the "*" symbol.<br><br>
# To demonstrate, issue the following Linux 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 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>
