Regular expression notation

a ordinary character, stands for itself

? the empty string

another way to write the empty string

M | N alternation

M • N concatenation

M* repetition (zero or more times)

M+ repetition (one or more times)

M? Optional, zero or one occurrence of M

[a-zA-Z] Character set alternation

. Period stands for any single character except newline

"a.+*" quotation, string stands for itself

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