Re: [Boost-users] Regex question

From: Eric Niebler (eric_at_[hidden])
Date: 2006-04-02 20:49:08

Kyle Alons wrote:
> std::string in("hello");
>  std::string exprStr("(.*)");
>  std::string replStr("goodbye$1");
> 
>  boost::regex expr(exprStr);
>  std::string out(boost::regex_replace(in, expr, replStr));
> 
>  printf("%s", out.c_str());
> 
> In Boost 1.31, this outputs "goodbyehello".
> 
> In 1.32+, it outputs "goodbyehellogoodbye".
> 
> Is it a bug, a by-design breaking change, or?  Thanks.

By design. The old behavior was buggy. By default, regex_replace 
substitutes globally, and your pattern matches the string in two places: 
first, it matches the whole string; second, it matches the empty 
sequence at the end. In perl:

$str = 'hello';
$str =~ s/(.*)/goodbye$1/g;
print "$str\n";

This prints "goodbyehellogoodbye". Have a look in the docs for 
regex_constants::format_first_only.

HTH,

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com