Regular Languages

• Finite State Machines with and without Output (Mealy Machines)
• Maths for Regular Expressions
• Regular Expressions
• Regular Languages
• Backus-Naur Form for Context-Free Languages

Regular Expressions

A regular expression is an expressions used to specify a set of strings that satisfy given conditions (A sequence of characters that define a search pattern). They are used by programmers and computers to work with text patterns. The most popular implementations are in search engines and search & replace dialogues in word processors.

Most programming languages support regular expressions, but the most common symbols are described below:

• | A vertical bar separates alternatives e.g. (D|d)is(c|k)
•  ? A question mark indicates zero or one of the preceding element e.g. colou?r
• * An asterisk indicates zero or more of the preceding element e.g. aa and aabbb would match aab*
• + A plus indicates one or more of the preceding element e.g. abc and abccc would match abc+

If your programming language supports regular expressions, such as Python, the syntax might be different.