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正則表示法 Regular Expression

正則表示法 Regular Expression. Regular Expression. A language is regular, if its syntax can be expressed by a single EBNF expression. The requirement that a single equation suffices also implies that only terminal symbols occur in the expression.

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正則表示法 Regular Expression

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  1. 正則表示法Regular Expression

  2. Regular Expression • A language is regular, if its syntax can be expressed by a single EBNF expression. • The requirement that a single equation suffices also implies that only terminal symbols occur in the expression. • Such an expression is called a regular expression. 2

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  4. Tokens • Identifiers: x y11 elsex • Keywords: if else while for break • Integers: 2 1000 -20 • Floating-point: 2.0 -0.0010 .02 1e5 • Symbols: + * { } ++ << < <= [ ] • Strings: “x” “He said, \”I love EECS 483\”” • Comments: /* bla bla bla */

  5. How to Describe Tokens • Use regular expressions to describe programming language tokens! • A regular expression (RE) is defined inductively • a ordinary character stands for itself •  empty string • R|S either R or S (alteration), where R,S = RE • RS R followed by S (concatenation) • R* concatenation of R 0 or mor times (Kleene closure)

  6. Language • A regular expression R describes a set of strings of characters denoted L(R) • L(R) = the language defined by R • L(abc) = { abc } • L(hello|goodbye) = { hello, goodbye } • L(1(0|1)*) = all binary numbers that start with a 1 • Each token can be defined using a regular expression

  7. RE Notational Shorthand • R+ one or more strings of R: R(R*) • R? optional R: (R|) • [abcd] one of listed characters: (a|b|c|d) • [a-z] one character from this range: (a|b|c|d...|z) • [^ab] anything but one of the listed chars • [^a-z] one character not from this range

  8. Regular Expression, R a ab a|b (ab)* (a| )b digit = [0-9] posint = digit+ int = -? posint real = int ( | (. posint)) = -?[0-9]+ (|(.[0-9]+)) Strings in L(R) “a” “ab” “a”, “b” “”, “ab”, “abab”, ... “ab”, “b” “0”, “1”, “2”, ... “8”, “412”, ... “-23”, “34”, ... “-1.56”, “12”, “1.056”, ... Note, “.45” is not allowedin this definition of real Example Regular Expressions

  9. Class Problem • A. Whats the difference? • [abc] abc • Extend the description of real on the previous slide to include numbers in scientific notation • -2.3E+17, -2.3e-17, -2.3E17

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