What are Regular Expressions?
A powerful notation for describing patterns in text
In arithmetic, we can use operations like + and × to build up expressions such as (5+3) × 4. Similarly, we can use the regular operations to build up expressions describing languages, which are called regular expressions.
A regular expression is a pattern that describes a set of strings. The value of a regular expression is a language - the set of all strings that match the pattern.
Regular expressions have an important role in computer science applications:
- Text search and manipulation
- Input validation in forms
- Lexical analysis in compilers
- Data extraction and transformation
- Pattern matching in programming languages
Regular Expression to Automaton
(a|b)*abbThompson's
Construction
Construction
NFA
Subset
Construction
Construction
DFA
Everyday Example
When you search for a specific pattern in a text editor or validate an email address in a web form, you're likely using regular expressions behind the scenes.
For example, the regular expression ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ can be used to match valid email addresses.