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Top Forums UNIX for Dummies Questions & Answers What's the Diff Between These Two Regexes? Post 302653849 by sudon't on Sunday 10th of June 2012 07:33:36 PM
Old 06-10-2012
What's the Diff Between These Two Regexes?

Trying to understand what's happening here, but I cannot figure it out.
I'm reading Mastering Regular Expressions, by Friedl, and he uses this as an example of how to grab quoted text:
Code:
egrep -o '"[^"]*"' ~/File.txt

...should pull in any quoted phrases. Match a literal double-quote, match anything not a double-quote until you hit the next literal double-quote.
But, he says [^"]* can match a newline, thereby returning quoted text even if it crosses lines. If you want to keep it from crossing lines, you should use this:
Code:
egrep -o '"[^"\n]*"' ~/File.txt

But this is where my head starts to hurt because a star should never fail, right? In other words, if it hits a newline, isn't a newline zero (or more) occurrences of 'not a newline', thereby allowing the regex to keep chugging along?
But looking at the difference between what each regex returns, the differences don't seem to have anything to do with newlines. Check out the different results each regex pulls from this snippet:
Quote:
those with whom he was most pleased. Having asked one Zeno, upon his
using some far-fetched phrases, "What uncouth dialect is that?" he
replied, "The Doric." For this answer he banished him to Cinara [354],
Code:
egrep -o '"[^"]*"' ~/File.txt
"What uncouth dialect is that?"
"The Doric."

egrep -o '"[^"\n]*"' ~/File.txt
"The Doric."

Why does the second regex miss the first quote? The first regex returns about twice as many hits as the second one, and they all appear to be valid, single line quotes.
I should also mention that I'm not trying to accomplish anything. My interest is purely academic, and I'm a total noob.

GNU grep
OS X 10.6.8
Original file is plain vanilla ASCII, each line ends in a newline.
 

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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)
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