Regular expressions (RegEx) allow for complex pattern matching in text. They use special characters to match wildcards, quantifiers, character classes and groupings. RegEx are used in programming languages and tools like Perl, sed, awk, Grep, and Vi. They are useful for string handling, manipulation and searching large amounts of text when looking for specific patterns rather than exact matches.

1. Regular Expressions
What are they all about?

2. What Are Regular Expressions?
• A RegEx (regular expression) is a special string for describing a pattern.
• Template for deterring matching
• Wild card on steroids
• Exquisite string handling and manipulation
• RegEx aren’t merely a part of Perl
• sed
• awk
• Grep
• Vi
• Search engines

3. RegEx Basics
• Code: /expression/
• The expression you are searching, matching processing is placed within forward slashes.
• Ex. /abc/, /[0-9]/, [A-Z], [a-cw-z] etc.
• When performing pattern matches you are primarily concerned with the final
yes(true) or no(false) outcome.
• Condtional based statements
• if
• while
• However, it is possible to extract the search string or line from text.

4. The Binding Operator
• By default RegEx expressions are tested against
the default variable in Perl, $_.
• Ex. $_ = “they”;
if(/hey/)
{
print(“Found it”);
}
• $_ is just the default but how do we
compare variables/strings to RegEx
expressions?!?!
• Enter the binding operator, =~.
• The binding operator tells Perl to match the string on the
left to the expression on the right.
• Ex. $name = “My name is Jason”;
if($name =~ /Jason/)
{
print(“That’s me!”);
}
• Ex. $isMe = (<STDIN> =~ /Jason);
if($isMe)
{
print(“It’s Jason”);
}

5. Metacharacters
• The power of regular expressions is derived from the power to NOT
search/filter through exact matches.
• Ex. Forensic Analysis
• Metacharacters are a number of special characters that have unique pattern
recognition in regular expression.
• Question: What type of metacharacters have you utilized on the command prompt?

6. Presenting the Metacharacters
• wildcard character
• Matches any
character except a
newline
• Ex. /d.g/
• Quantifier
• Match the
preceding item 0 or
more times
• Ex. /du*de/
• Quantifier
• Match the
preceding item 1
or more times
• Ex. /du+de/
• Quantifier
• Math the preceding
item 0 or 1 times.
• Optional
• Ex. /dude!?/
*. + ?
• Also the escape sequence characters work in RegEx as well, n, t, r, , .

7. Grouping in Patterns
• Parentheses can be used to group parts of a pattern.
• Ex. /(franko)+/ /(franko)*/ /(franko +jason)/
• However the more beneficial technique is that parentheses form capture
groups.
• With a capture group created you can reuse them using back references.
• Code: g{ref#}
• Ex. $_ = “zippiddy doo daa”;
• /(.)g{1}/ /(.)(i)g{1}g{1}/ /(.)(.)g{-1}/ /i(.)g{1}y/