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Questions about regular expressions, a formalism to describe regular languages.

Questions about regular expressions, a formalism to describe regular languages.

Note that regular expressions in a computer science context are not the same as POSIX RegExp or similar. A regular expression in the formal language sense $R$ can be one of

• the empty language $\emptyset$,
• the empty string $\varepsilon$ (also $\lambda$),
• an individual symbol $a \in \Sigma$ with $\Sigma$ some finite alphabet,
• a concatenation $R_1 \cdot R_2$,
• an alternative $(R_1 \mid R_2)$ (often $+$ is used) and
• a Kleene star $(R_1)^*$.

See the corresponding Wikipedia article for details.

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