01-13-2004
Much of the real power behind many of the tools in UNIX comes from an understanding of "regular expressions".
A coulple of nice sites to visit and learn from:
http://www.regular-expressions.info/
http://en2.wikipedia.org/wiki/Regular_expression
A little history (from wikipedia):
A regular expression (abbreviated as regexp or regex) is a string that describes a whole set of strings, according to certain syntax rules. These expressions are used by many text editors and utilities (especially in the Unix operating system) to search bodies of text for certain patterns and, for example, replace the found strings with a certain other string.
The origin of regular expressions lies in automata theory and formal language theory (both part of theoretical computer science). These fields study models of computation (automata) and ways to describe and classify formal languages. A formal language is nothing but a set of strings. In the 1940s, Warren McCulloch and Walter Pitts described the nervous system by modelling neurons as small simple automata. The mathematician, Stephen Kleene, later described these models using his mathematical notation called regular sets. Ken Thompson built this notation into the editor qed, then into the Unix editor ed and eventually into grep. Ever since that time, regular expressions have been widely used in Unix and Unix-like utilities such as: expr, awk, Emacs, vim, lex, and Perl. Most tools use an implementation of the regex library built by Henry Spencer.
Cheers,
Keith
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LEARN ABOUT ULTRIX
re_exec
regex(3) Library Functions Manual regex(3)
Name
re_comp, re_exec - regular expression handler
Syntax
char *re_comp(s)
char *s;
re_exec(s)
char *s;
Description
The subroutine compiles a string into an internal form suitable for pattern matching. The subroutine checks the argument string against
the last string passed to
The subroutine returns 0 if the string s was compiled successfully; otherwise a string containing an error message is returned. If is
passed 0 or a null string, it returns without changing the currently compiled regular expression.
The subroutine returns 1 if the string s matches the last compiled regular expression, 0 if the string s failed to match the last compiled
regular expression, and -1 if the compiled regular expression was invalid (indicating an internal error).
The strings passed to both and may have trailing or embedded newline characters; they are terminated by nulls. The regular expressions
recognized are described in the manual entry for given the above difference.
Diagnostics
The subroutine returns -1 for an internal error.
The subroutine returns one of the following strings if an error occurs:
No previous regular expression
Regular expression too long
unmatched (
missing ]
too many () pairs
unmatched )
See Also
ed(1), ex(1), egrep(1), fgrep(1), grep(1)
regex(3)