Pattern Matching: search(), match() & Character Sets

Now that you understand the basic symbols of RegEx, let us explore how Python actually searches through text and returns Match Objects.

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1. re.search() vs. re.match() vs. re.fullmatch()

Many beginners get confused between these three functions. Here is the clear distinction:

Function	What it does	Where it looks
re.search()	Scans the entire string and returns the first match anywhere	Anywhere in text
re.match()	Checks if the pattern matches strictly at the very start of the string	First index only
re.fullmatch()	Checks if the pattern matches the entire complete string from start to finish	Whole string

import re

message = "Order ID: #98421 confirmed."

# 1. re.search: Finds #98421 anywhere in the text
search_result = re.search(r"#\d+", message)
print("search found:", search_result.group())  # Output: search found: #98421

# 2. re.match: Fails because the string starts with 'Order', not digits!
match_result = re.match(r"#\d+", message)
print("match found:", match_result)  # Output: match found: None

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2. The Match Object: Extracting Positions and Values

When re.search() finds a match, it returns a Match Object with useful methods:

import re

text = "Flight 6E-204 is on time."
match = re.search(r"\d[A-Z]-\d+", text)

if match:
    print("Matched Text:", match.group())  # Output: 6E-204
    print("Start Index:", match.start())    # Output: 7
    print("End Index:", match.end())        # Output: 13
    print("Span Coordinates:", match.span()) # Output: (7, 13)

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3. Character Classes [...]

A character set in square brackets [...] matches any single character inside the brackets:

# [aeiou]  --> Matches any lowercase vowel
# [a-z]    --> Matches any lowercase letter from a to z
# [A-Z]    --> Matches any uppercase letter from A to Z
# [0-9]    --> Matches any digit from 0 to 9
# [^0-9]   --> The caret ^ INSIDE brackets means NOT a digit (Negation)

Example: Matching Hex Color Codes

import re

color_code = "#A3F90B"
is_valid_hex = bool(re.search(r"^#[0-9A-Fa-f]{6}$", color_code))
print(f"Valid Hex Color? {is_valid_hex}")  # Output: True

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4. Special RegEx Character Shortcuts

Instead of typing [0-9] or [a-zA-Z0-9_], Python provides clean built-in shortcuts:

Shortcut	Matches	Meaning
\d	[0-9]	Any single digit
\D	[^0-9]	Any non-digit character
\w	[a-zA-Z0-9_]	Any alphanumeric word character + underscore
\W	[^a-zA-Z0-9_]	Any non-word symbol (e.g., spaces, punctuation)
\s	[ \t\n\r]	Any whitespace (space, tab, newline)
\S	[^ \t\n\r]	Any non-whitespace character

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Quick Summary

• Character Classes ([...]): Define a set of allowed characters (e.g., [a-z], [0-9]).
• Negated Classes ([^...]): Matches any character not in the set.
• Character Shortcuts: \d (digits), \w (word chars), \s (whitespace) and their uppercase inverses (\D, \W, \S).
• Match Anchors: ^ anchors to start of string; $ anchors to end of string.

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What's Next?

Let's look at how to extract multiple matching items, emails, and phone numbers from documents in Extracting Data with findall() & sub()!