Working with Regular Expressions

Note

• Regular expressions, often abbreviated as regex or regexp, are patterns used to match character combinations in strings.
• They are useful for searching, validating, extracting, or replacing text.
• A regular expression is a sequence of characters that defines a search pattern.

🧠 How DoThey Work?

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A regular expression defines a pattern. That pattern is then compared to a target string, and based on the rules, it either:

• Matches the pattern (success),
• Or doesn’t match (failure).

Why Use Regular Expressions?

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• Validation: Email addresses, phone numbers, passwords
• Text extraction: Pulling data from unstructured text
• Search and Replace: Complex find/replace operations
• Text Processing: Cleaning, transforming, and parsing text
• Data Scraping: Extracting specific information from documents

📐 Regex Patterns

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Basic Regex Patterns

Character	Meaning
.	Matches any character except newline
\d	Matches any digit (0-9)
\w	Matches any word character (alphanumeric + underscore)
\s	Matches any whitespace character
\D, \W, \S	Negated versions (non-digit, non-word, non-whitespace)

Quantifiers

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Symbol	Meaning
*	0 or more times
+	1 or more times
?	0 or 1 time (optional)
{n}	Exactly n times
{n,}	n or more times
{n,m}	Between n and m times

Character Classes

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• Square brackets [ ] define a character class.
• Match any single character from the specified set.
• Examples:
  • [aeiou] - matches any vowel
  • [0-9] - matches any digit (same as \d)
  • [a-zA-Z] - matches any letter (upper or lowercase)
  • [^0-9] - matches any character that’s NOT a digit

Groups and Ranges

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Pattern	Description
(abc)	Capturing group
(?:abc)	Non-capturing group
[abc]	a, b, or c
[^abc]	Not a, b, or c
[a-z]	Range (lowercase)
[A-Z]	Range (uppercase)

Alternation

/yes|no/

js

Matches either "yes" or "no"

Anchors and Boundaries

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Symbol	Meaning
^	Start of string or line
$	End of string or line
\b	Word boundary
\B	Not a word boundary

🚩 Regex Flags/Modifiers

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Flag	Description
g	Global search (find all matches)
i	Case-insensitive search
m	Multi-line mode (^ and $ match line start/end)
s	Allows . to match newline characters
u	Unicode mode
y	Sticky search (matches from lastIndex)

Example:

const regex = /hello/gi;

js

Groups and Capturing

• Parentheses ( ) create capture groups
• Used to:
  • Apply quantifiers to entire sequences
  • Extract specific parts of the match
  • Reference matched text with backreferences

# Example: Capturing name parts
pattern = r"(\w+)\s(\w+)"
text = "John Smith"
# Captures: Group 1 = "John", Group 2 = "Smith"

plaintext

Alternation and Non-capturing Groups

• | pipe symbol for alternation (OR operator)
• (?:...) for non-capturing groups
• Examples:
  • cat|dog matches “cat” or “dog”
  • I love (cats|dogs) matches “I love cats” or “I love dogs”
  • (?:https?|ftp):// matches “http://”, “https://”, or “ftp://“

Lookahead and Lookbehind

• Lookahead: (?=...) positive, (?!...) negative
• Lookbehind: (?<=...) positive, (?<!...) negative
• Zero-width assertions (don’t consume characters)
• Examples:
  • \w+(?=\s) - word followed by whitespace
  • (?<=\$)\d+ - digits preceded by dollar sign
  • \b\w+\b(?!\s+and\b) - word NOT followed by “and”