Regular languages, Kleene star

Regular Expressions

Master pattern matching and regular language representation

What are Regular Expressions?

Regular Expressions (RegEx) are algebraic expressions that describe regular languages. They provide a concise and flexible means for matching strings of text, such as particular characters, words, or patterns of characters.

Regular expressions are built using basic symbols and operators:

  • Basic symbols: Characters from the alphabet
  • Union (|): Matches either expression
  • Concatenation: Matches expressions in sequence
  • Kleene Star (*): Matches zero or more repetitions
  • Parentheses (): Groups expressions

Regular Expression operators

Operator Symbol Description Example
Union | Either expression a|b
Concatenation · Sequence of expressions ab
Kleene Star * Zero or more repetitions a*
Plus + One or more repetitions a+
Optional ? Zero or one occurrence a?
Grouping () Groups expressions (ab)*

Common Regular Expression Examples

Email Pattern
[a-zA-Z0-9]+@[a-zA-Z0-9]+\.[a-zA-Z]{2,}

Matches basic email addresses

Example: user@domain.com
Phone Number
\d{3}-\d{3}-\d{4}

Matches US phone number format

Example: 123-456-7890
Binary Strings
(0|1)*

Matches any binary string

Example: 101010, 001, 1
Even Length Strings
((0|1)(0|1))*

Matches binary strings of even length

Example: 01, 1010, 0011
Strings Ending with 'ab'
(a|b)*ab

Matches strings ending with 'ab'

Example: aab, bab, aaab
At Least One 'a'
b*ab*|a(a|b)*

Matches strings with at least one 'a'

Example: a, ba, ab, aab

Visual Pattern Builder

Build regular expressions visually by combining components

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Enhanced RegEx Tester

Test strings with visual matching and step-by-step execution

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Pattern Statistics

Pattern Length: 0
operators Used: 0
Groups: 0
Tests Run: 0
Matches Found: 0
Pattern Complexity
Simple pattern with basic operators
Quick Test Patterns
Basic Patterns
a*
Zero or more 'a'
a+
One or more 'a'
a?
Optional 'a'
(a|b)*
Any combination of a and b
Advanced Patterns
(a|b)*ab
Strings ending with 'ab'
(01|10)*
Alternating binary
[0-9]+
One or more digits
\w+@\w+\.\w+
Basic email pattern

Visual Pattern Representation

Batch Testing & Analysis

Test multiple strings and analyze pattern matching behavior

Pattern Converter & Generator

Convert between different regex formats and generate patterns

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