[Boost-users] [regex] ".*" expression matches twice in regex_replace?

From: Arne Babnik (boost_at_[hidden])
Date: 2007-08-01 12:13:45

Hi all,

I am trying to perform some simple (as I thought) text manipulation.
The task was to prepend a text with some other, e.g. 1234 -> 01234. I  
tried to do this with the expression ".*" and format "0$&".

I used regex_replace with the following parameters:

std::basic_string<char>  string("1234");
std::basic_string<char>  format("0$&");
boost::regex             exp(".*");
std::basic_string<char>  result;

result = boost::regex_replace(string, exp, format);

I was very surprised when I got the result "012340", i.e. the format  
string was applied twice. After some testing, I found out that  
regex_replace matches a second time at the end of the string, where  
the expression matched an empty string (therefore only the zero was  
appended at the end).

When using the (undocumented) option match_not_initial_null, it  
basically works - but then empty input strings would not match either.  
This also applies to the expression ".+", which works correct on the  
input but would not match on empty strings as well.
The only solution I found so far was using "^.*" as expression.

Is this a bug or a feature? Or what did I get wrong with this regular  
expression?

Regards,
Arne

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