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Lab 8: Regular Expressions

Learn the world of Black Magic with Regular Expressions - symbols expressing patterns in texts. Explore grep commands and common regex syntax using characters like *, +, [a-z], and more to search efficiently. Real-world examples included!

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Lab 8: Regular Expressions

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  1. Lab 8: Regular Expressions Enter the world of Black Magic

  2. What is a regular expression? • A regular expression is a sequence of symbols and characters expressing a string or pattern to be searched for within a longer piece of text • Or in theory of computation terms: A regular expression is an algebraic formula whose value is a pattern consisting of a set of strings, called the language of the expression [This is the confusion definition that we don’t need to learn]

  3. Examples Files on GitHub • We will be using the same files from Lab 7 to learn more about regex in this lab

  4. Grep • Grep is the most common regex command used on the command line • Grep stands for global regular expression print • The syntax is grep [options] pattern [file] • In essence, you define a search pattern and a file and then the shell will read through the file to find instances of that pattern

  5. Common Regex Syntax • The following are the various characters that are used in a regex: • . (dot) – a single character • ? - preceding character matches zero or one times • * - preceding character matches zero or more times • + - preceding character matches one or more times • [a-z] – character is included in the range of characters specified by the brackets • [^ a-z] – character is not one of those included in the brackets in that range • {n} – preceding character matches exactly n times • {n, m} – preceding character matches at least n times, but not more than m times • ^ – matches at the beginning of the line • $ – matches at the end of the line

  6. Grep Examples • grep ‘firefox’ ps-output.txt • grep ‘libreoffice’ ps-output.txt • grep ‘f.x’ period.txt • grep ‘…..’ period.txt • Grep -x ‘[a-z]\+’ random-passwords.txt • grep -x ‘[0-9]\+’ random-passwords.txt • grep ‘[a-c]\+’ random-passwords.txt • Grep ‘[T]$’ random-alphanumeric.txt • Grep ‘^y.*[0-9]’ random-alphanumeric.txt

  7. Other Commands • There are other numerous commands that you can use other than grep to filter through files • Comm • Awk • Sed [this is very complicated]

  8. Real World Example • In Spring 2018, I was tasked with making an anti-virus which could detect malicious files • I used various filtering commands and several regex commands in order to filter out unnecessary inputs and keep the desired input. • Code:

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