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What is, in the strict sense of the word, a "regular expression"?
a match expression as regulated by ANSI X3.159-1989
an expression describing a Chomsky Type-3 Grammar
an expression describing a Context-Free Grammar
a programming construct peculiar to Perl
An expression describing a Chomsky Type-3 Grammar (aka Regular Grammar, hence the name...)
(ANSI X3.159-1989 describes original ANSI-C and makes no mention of regexps; a type-2 grammar [aka context-free grammar] cannot be described by a FSM or a regular expression, you need a BNF or similar; Perl is the obvious nonense-answer)