Introduction to Regular Expressions (RegEx)

Have you ever tried to find all phone numbers in a 500-page PDF document, or verify if an email address entered by a user contains @ and .com?

Using standard string methods like .find() or .startswith() becomes nearly impossible when text patterns are complex and dynamic.

A Regular Expression (RegEx) is a specialized, compact search language designed to match, extract, and clean patterns in text.

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1. The Real-World Metaphor: The Metal Detector

Think of standard string searching ("sai" in text) like looking for one specific person wearing a red t-shirt.

A Regular Expression is like a metal detector at a stadium gate. It doesn't look for a specific key; it beeps for anything that matches the metal pattern (coins, keys, belt buckles).

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2. Python's Standard re Module & Raw Strings

Python has a built-in library for regular expressions called re:

import re

Why You Must Always Use Raw Strings (r"...")

In regular Python strings, a backslash \ is used for special escape characters like \n (newline) and \t (tab).

In RegEx, backslashes are also used for special patterns (like \d for digits). To avoid Python confusing \n with a regex rule, always prefix regex patterns with r:

# ❌ Confusing to Python's parser:
pattern = "\\d+"

# ✅ Clean and standard raw string:
pattern = r"\d+"

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3. Essential RegEx Metacharacters

Metacharacters are special symbols with unique superpowers in regular expressions:

Symbol	Meaning	Example Pattern	Matches
. (Dot)	Any single character (except newline)	r"c.t"	cat, cot, c9t
^ (Caret)	Starts with	r"^Hello"	"Hello world" (at start only)
$ (Dollar)	Ends with	r"end$"	"The end" (at end only)
* (Star)	0 or more repetitions	r"go*l"	gl, gol, goool
+ (Plus)	1 or more repetitions	r"go+l"	gol, goool (not gl)
? (Question)	0 or 1 optional appearance	r"colou?r"	color, colour
**`	`** (Pipe)	OR condition	`r"cat

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4. Quick Example: Checking if a Pattern Exists

import re

text = "Payment of Rs 4500 received via UPI"

# Check if text contains the word 'UPI'
if re.search(r"UPI", text):
    print("✅ Transaction verified via UPI!")
else:
    print("❌ Other payment method.")

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Quick Summary

• Regular Expression (RegEx): A sequence of characters defining a search pattern to validate, extract, or replace text.
• import re: Python's built-in regular expression engine.
• Raw Strings (r"..."): Always prefix regex patterns with r to preserve backslashes (\d, \n) without Python escape interference.
• re.search(pattern, text): Scans through a string looking for the first location where the pattern matches.

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What's Next?

Let's learn the core regex syntax, metacharacters, and character classes in Pattern Matching & Character Classes in the next lesson!