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→INVESTIGATION 1: SIMPLE & COMPLEX REGULAR EXPRESSIONS
# 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]$" textfile1.txt | more</span><br><br>The '''*''' complex regular expression symbol is often confused with filename expansion. In other words, it does NOT represent zero or more of '''any character''', but zero or or occurrences of the character that comes before the * symbol.<br><br>
# To demonstration, issue the following command to display zero or more occurrences of the letter x:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "x*" textfile1.txt | more</span><br><br>You will most likely notice most lines of the file is displayed.<br><br>
# Let's issue a command to display strings that contain more than one occurrence of the letter x:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "xx*" textfile1.txt | more</span><br><br>Why did this work? because the pattern indicates one occurrence of the letter x, followed by zero or MORE occurrences of the letter x.<br><br>If you combine the complex regular expression symbols .* it will act like zero or more occurrence of any character (like * did in filename expansion).<br><br># Issue the following command to match strings begin and end with a number with nothing or anything inbetween:<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[0-9].*[0-9]$" textfile1.txt | more</span><br><br># x
=INVESTIGATION 2: EXTENDED REGULAR EXPRESSIONS =
