[Boost-users] Regex: Question about greedy/non-greedy precedence

From: Arne Babnik (boost_at_[hidden])
Date: 2008-01-04 18:55:02

Hello,

I'm using boost::regex 1.34.0 and experience behaviour with mixed  
greedy and non-greedy operators in the same expression which I do not  
understand.

I have either one of the following input strings:
user1_at_[hidden]
user2

and want to keep only the usernames, so I use the following expression:
(.*?)(@.*)?
with the format string $1.

I would expect that basically the regex machine would first try to  
satisfy the greedy operator "?", and then fill up the non-greedy "*?"  
with the remaing part of the input.

However, the following does NOT work (the hostname is not removed):
std::string output;
output = boost::regex_replace(std::string("user1_at_[hidden]"),
                               boost::regex("(.*?)(@.*)?"),
                               std::string("$1"),
                               boost::regex_constants::format_first_only);

However, if I omit the "format_first_only", it works as expected.

What do I miss?

Thanks,

Arne

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