AlgoMaster

Engineering Solutions

Regular Expression Matcher

Test and validate regular expressions against multiple input strings with detailed matching analysis

Regular Expressions (Regex)

Regular expressions are formal language patterns used to match and manipulate strings. They provide a powerful and concise way to search, match, and replace text patterns.

Theory & Concepts

Definition:

A regular expression is a sequence of characters that defines a search pattern for strings. They are formally equivalent to finite automata and define regular languages.

Key Points:

  • Metacharacters define special matching behaviors (* + ? . | [ ] ^ $)
  • Character classes allow matching sets of characters
  • Quantifiers specify how many times to match
  • Equivalent to finite automata in computational power

Applications:

  • Text search and pattern matching in editors
  • Data validation (email, phone numbers, URLs)
  • Log file analysis and data extraction
  • Programming language lexical analysis

University Guidelines

Exam Weightage
6-8 marks Marks

Syllabus Coverage:

  • Regular expression definition and syntax
  • Metacharacters and quantifiers
  • Character classes and ranges
  • Equivalence with finite automata
  • Converting regex to NFA and DFA

Common Questions:

  • Design regex for given language patterns
  • Convert regular expression to finite automaton
  • Test strings against regular expressions
  • Optimize regular expressions for efficiency

Exam Tips:

Practice constructing regex for common patterns
Understand the equivalence with finite automata
Know how to convert between regex and NFA/DFA
Remember precedence rules for operators

Algorithm Details

O(n*m) where n=text length, m=pattern length
Time Complexity
O(m) for NFA states
Space Complexity

Algorithm Steps:

  1. 1Parse the regular expression pattern
  2. 2Build NFA using Thompson's construction
  3. 3Test input string against the automaton
  4. 4Return match result and captured groups

Regular Expression

Test Strings

abb
aabb
babb
ababb
ab
ba
abba

Pattern Analysis

Current Pattern:

(a|b)*abb

Regular Expression Reference

Basic Operators:

. - Any single character
* - Zero or more repetitions
+ - One or more repetitions
? - Zero or one occurrence
| - Alternation (OR)

Character Classes:

[abc] - Any of a, b, or c
[a-z] - Any lowercase letter
[0-9] - Any digit
\d - Digit character
\w - Word character

Example Patterns:

ab* - "a" followed by zero or more "b"s
(ab)+ - One or more "ab" sequences
a|b - Either "a" or "b"
^a.*b$ - Starts with "a", ends with "b"