Learning Regular Expressions

Regular Expressions are shortcode langauge settings for computer programming used in cybersecurity. This was a LinkedIn Course that I did as part of my Masterschool studies

According to Grok:

Regular expressions (often abbreviated as regex or regexp) are a powerful, concise way to describe and match patterns in text. They are essentially a mini-programming language for searching, validating, extracting, or replacing strings based on specific rules.

Common Symbols & Syntax

Symbol	Meaning	Example	Matches
.	Any single character (except newline)	c.t	cat, cot, c3t
*	Zero or more of the previous	ca*t	ct, cat, caat, caaat
+	One or more of the previous	ca+t	cat, caat, caaat (but not ct)
?	Zero or one of the previous	ca?t	ct, cat
^	Start of string/line	^Hello	“Hello world” ✓, “Say Hello” ✗
$	End of string/line	world$	“Hello world” ✓
[]	Character class (any one inside)	[aeiou]	any vowel
[^]	Negated class	[^0-9]	any non-digit
|	OR	cat|dog	cat or dog
()	Grouping / capture	(ab)+	ab, abab, ababab
\d	Digit [0-9]	\d{3}	123, 456
\w	Word character [a-zA-Z0-9_]	\w+	hello, var123
\s	Whitespace (space, tab, etc.)	\s+	one or more spaces
{n}	Exactly n times	\d{3}	exactly 3 digits
{n,}	n or more	\d{2,}	2+ digits
{n,m}	Between n and m	\d{2,4}	2–4 digits

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Real-World Examples

Goal	Regex Pattern	Matches
Find emails	[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}	[email protected]
US phone (XXX-XXX-XXXX)	\d{3}-\d{3}-\d{4}	123-456-7890
HTML tag	<[^>]+>	<p>, <div class=”x”>
Date (YYYY-MM-DD)	\d{4}-\d{2}-\d{2}	2025-10-31
Extract numbers	\d+	all sequences of digits

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Where Are Regex Used?

• Programming: Python (re module), JavaScript, Java, PHP, etc.
• Tools: grep, sed, awk, VS Code search, Notepad++, Google Analytics
• Validation: Forms, password strength, input sanitization

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Example in Python

python

import re

text = "Contact: [email protected] or [email protected]"
pattern = r'[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}'

emails = re.findall(pattern, text)
print(emails)  # ['[email protected]', '[email protected]']