Regular expressions provide a concise and flexible means for identifying strings of text of interest, such as particular characters, words, or patterns of characters.

Regular expressions (abbreviated as regex or regexp) are written in a formal language that can be interpreted by a regular expression processor, a program that either serves as a parser generator or examines text and identifies parts that match the provided specification.

A regular expression is an expression that describes a set of strings.

It is composed of Symbols and Operators.

A regular expression over S is an expression formed using the following rules:

The symbol Φ is a regular expression

The symbol ε is a regular expression

For every a E Σ, the symbol a is a regular expression

If R and S are regular expressions, so are RS, R+S and R*.

Examples

01* = {0, 01, 011, 0111, ...}

(01*)(01) = {001, 0101, 01101, 011101, ...}

(0+1)*

(0+1)*01(0+1)*

((0+1)(0+1)+(0+1)(0+1)(0+1))*

((0+1)(0+1))*+((0+1)(0+1)(0+1))*

(1+01+001)*(ε+0+00)

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