Boost Regex regex_match (grep option)

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Hi,

Environment: Windows 7 (32 bit), Visual Studio 2010 (Visual C++)

I am trying to use Boost REGEX regex_match algorithm.
I am using its grep flag.
My code is as follows and here (a)* matches string "aaa", but still b comes out to be false.

bool MatchWithGrep(string& pattern, string& data)
{

	const boost::regex e(pattern.c_str(), boost::regex::grep);
	return regex_match(data.c_str(), e);

}

I am passing following parameters

bool b = MatchWithGrep("(a)*\\n123", "aaa");

Can any one help me to find whats wrong with my code?
Thanks in advance.

Posted 25-Dec-13 22:41pm

Member 10329703

Updated 26-Dec-13 5:58am

Albert Holguin

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OriginalGriff · Answer 1 · 2013-12-25T22:50:00

Solution 1

No - but your data and the pattern do not match.
You are searching for "zero or more 'a' followed by a newline character and the string '123'" which is not present in your sample data of "aaa"
Try calling it this way:

C++

bool b = MatchWithGrep("(a)*", "aaa\\n123");

And it might work better...

Posted 25-Dec-13 22:50pm

OriginalGriff

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Member 10329703 26-Dec-13 5:29am

OriginalGriff,

In Boost Help, following is documented.

Link -
http://www.boost.org/doc/libs/1_55_0/libs/regex/doc/html/boost_regex/syntax/basic_syntax.html

When an expression is compiled with the flag grep set, then the expression is treated as a newline separated list of POSIX-Basic expressions, a match is found if any of the expressions in the list match, for example:
boost::regex e("abc\ndef", boost::regex::grep);
will match either of the POSIX-Basic expressions "abc" or "def".

Considering this, in my example i have 2 basic expressions
(a)* and 123. Presence of any should give me bool b as true.
And my data is "aaa". So it should give true. But i am getting false.

OriginalGriff 26-Dec-13 5:59am

Ah! But that isn't what you have... Your code:
bool b = MatchWithGrep("(a)*\\n123", "aaa");
passes a pattern without a newline:
(a)*\n123
because the '\\' is escaped to a single backslash character.
Try
bool b = MatchWithGrep("(a)*\n123", "aaa");
Instead...

Member 10329703 26-Dec-13 7:01am

Since its c++ string \\n will be taken as \n internally.
And if i give "(a)*\n123" it will be read as "(a)*n123".
I had debugged the code.

OriginalGriff 26-Dec-13 7:05am

No, it won't.
"\\n" is two characters: a '\' character followed by a 'n' character. You need (from the sample in the article you linked to) a single '\n' character as it wants newline separated strings.
And "(a)*\n123" is two such strings: "(a)*" and "123" separated by a newline character.

Member 10329703 26-Dec-13 9:43am

I had tried this solution and had debugged the code, it shows value for pattern variable as "(a)*n123" in watch window. Also result comes out to be false for variable b.