Regex Set


Regular Expression $r$ represents a pattern of string, where
- the string set is called [Regular Language](https://texonom.com/regular-language-f3e86ff9669c45e6b32d63953fd6165b) 
- The character is called [Alphabet](https://texonom.com/alphabet-35edfc5226a74993ac4402b11f2ed835) 
- given an alphabet $\sum$, we can construct regex $r$
$$\text{if } r=a, a \text{ in } \sum. \text{ } L(r) = \{a\}$$
$$\text{if } r=ɸ, \text{ } \text{ } L(r) = \{\}  \text{ // empty set}$$
$$\text{if } r=ε, \text{ } L(r)=\{ε\} \text{ // a set with an empty string}$$











