Generation of Pattern - Matching Algorithms by Extended Regular Expressions

It is dicult to express the denition of the comments of C language in a regular expression. However, the denition can be expressed by a simple regular expression by introducing a special symbol, called the any-symbol, that represents any single character, or by introducing a kind of negation symbol into regular expressions. In general, the problem of string pattern matching can be expressed as such an extended regular expression, and the corresponding nite state automaton generated from the expression is equivalent to the Knuth-Morris-Pratt pattern-matching algorithm [4]. In particular, if we use the any-symbols, the pattern is not restricted to a string of characters. It can be any regular expression. Our method can also be applied to the problem of repeated pattern matching. The Aho-Corasick algorithm [3] can be derived mechanically from an extended regular expression that contains any-symbols. 1 Introduction The denition of a comment of C language can be given as a string composed of three strings in the following order: /* , a string that does not contain */ (may be an empty string), */. Expressing the above denition by a regular expression is complicated because it is dicult to express \a string that does not contain */." Here, */ means a string of length two composed of * followed by /. Although we can use the negation symbols in lex, a well-known scanner generator , the expression for the comment is still complex{for example, the repetition of the immediate predecessor zero or more times, and [^*/] expresses one character that is neither * nor /. This problem can be simply expressed if we can designate \a string that does not contain */" directly. If the expression of the above string is