-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of Alessandro Candini Sent: Friday, June 24, 2011 1:50 AM To: boost-users@lists.boost.org Subject: [Boost-users] [Regex] Emulate awk
I have a string like the following:
string myStr = "Monthly Ecosystem NPP = 0.360591 tDM/ha month";
How can I use boost regex to isolate 0.360591, as I would do in bash with a echo $myStr | awk '{ print $5 }' ? [snip]
Alessandro, The first shift to make is that with a RegEx you are dealing with recognizing character types, not delimited substrings as with awk. You could do it that way, by recognizing the 4 spaces and ignoring the stuff in-between. boost::regex e("^[^\\s]+\\s[^\\s]+\\s[^\\s]+\\s[^\\s]+\\s([^\\s]+)"); ... which will create a regex pattern e that starts at the beginning of the string, ignores four batches of non-space stuff followed by spaces, then captures the final non-space batch which is hopefully always your number. This is easier, just recognizing the batch of numbers with a decimal: boost::regex exp("([0-9.]+)"); ... will create a pattern called exp that will find some string of digits with periods. If you can be sure that there will only be one floating point non-negative number in the string this is all you need. You can also combine the two approaches. // code #include <stdlib.h> #include <boost/regex.hpp> #include <string> #include <iostream> using namespace boost; regex exp("[0-9.]+"); int process(std::string aString) { cmatch what; if (regex_match(aString, what, exp)) { return std::atof(what[1].first); } return -1; } // end code There are many books and web tutorials on Perl-compatible regular expressions, which are the default format. Once you get the hang of the C++ wrappings, you should be ready to study the RegEx syntax and apply it.