Matching and Extracting data using regular expression - c++

Question

Problem: To find a matching string and to extract data from the matched string. There are a number of command strings which has keywords and data.
Command Examples:
Ask name to call me
Notify name that do this action
Message name that request
Keywords: Ask, Notify, Message, to, that. Data:
Input strings:
Ask peter to call me
Notify Jenna that I am going to be away
Message home that I am running late
My problem consists of two problems
1) Find matching command
2) Extract data
Here is what I am doing:
I create multiple regular expressions:
"Ask[[:s:]][[:w:]]+[[:s:]]to[[:s:]][[:w:]]+" or "Ask([^\t\n]+?)to([^\t\n]+?)"
"Notify[[:s:]][[:w:]]+[[:s:]]that[[:s:]][[:w:]]+" or "Notify([^\t\n]+?)that([^\t\n]+?)"
void searchExpression(const char *regString)
{
std::string str;
boost::regex callRegEx(regString, boost::regex_constants::icase);
boost::cmatch im;
while(true) {
std::cout << "Enter String: ";
getline(std::cin, str);
fprintf(stderr, "str %s regstring %s\n", str.c_str(), regString);
if(boost::regex_search(str.c_str(), im, callRegEx)) {
int num_var = im.size() + 1;
fprintf(stderr, "Matched num_var %d\n", num_var);
for(int j = 0; j <= num_var; j++) {
fprintf(stderr, "%d) Found %s\n",j, std::string(im[j]).c_str());
}
}
else {
fprintf(stderr, "Not Matched\n");
}
}
}
I am able to Find a matching string, I am not able to extract the data.
Here is the output:
input_string: Ask peter to call Regex Ask[[:s:]][[:w:]]+[[:s:]]to[[:s:]][[:w:]]+
Matched num_var 2
0) Found Ask peter to call
1) Found
2) Found
I would like to extract peter and call from Ask Peter to call.

Answer 1

Since you're really wanting to parse a grammar, you should consider Boost's parser generator.
You'd simply write the whole thing top-down:
auto sentence = [](auto&& v, auto&& p) {
auto verb = lexeme [ no_case [ as_parser(v) ] ];
auto name = lexeme [ +graph ];
auto particle = lexeme [ no_case [ as_parser(p) ] ];
return confix(verb, particle) [ name ];
};
auto ask = sentence("ask", "to") >> lexeme[+char_];
auto notify = sentence("notify", "that") >> lexeme[+char_];
auto message = sentence("message", "that") >> lexeme[+char_];
auto command = ask | notify | message;
This is a Spirit X3 grammar for it. Read lexeme as "keep whole word" (don't ignore spaces).
Here, "name" is taken to be anything up to the expected particle¹
If you just want to return the raw string matched, this is enough:
Live On Coliru
#include <iostream>
#include <boost/spirit/home/x3.hpp>
#include <boost/spirit/home/x3/directive/confix.hpp>
namespace x3 = boost::spirit::x3;
namespace commands {
namespace grammar {
using namespace x3;
auto sentence = [](auto&& v, auto&& p) {
auto verb = lexeme [ no_case [ as_parser(v) ] ];
auto name = lexeme [ +graph ];
auto particle = lexeme [ no_case [ as_parser(p) ] ];
return confix(verb, particle) [ name ];
};
auto ask = sentence("ask", "to") >> lexeme[+char_];
auto notify = sentence("notify", "that") >> lexeme[+char_];
auto message = sentence("message", "that") >> lexeme[+char_];
auto command = ask | notify | message;
auto parser = raw [ skip(space) [ command ] ];
}
}
int main() {
for (std::string const input : {
"Ask peter to call me",
"Notify Jenna that I am going to be away",
"Message home that I am running late",
})
{
std::string matched;
if (parse(input.begin(), input.end(), commands::grammar::parser, matched))
std::cout << "Matched: '" << matched << "'\n";
else
std::cout << "No match in '" << input << "'\n";
}
}
Prints:
Matched: 'Ask peter to call me'
Matched: 'Notify Jenna that I am going to be away'
Matched: 'Message home that I am running late'
BONUS
Of course, you'd actually want to extract the relevant bits of information.
Here's how I'd do that. Let's parse into a struct:
struct Command {
enum class Type { ask, message, notify } type;
std::string name;
std::string message;
};
And let's write our main() as:
commands::Command cmd;
if (parse(input.begin(), input.end(), commands::grammar::parser, cmd))
std::cout << "Matched: " << cmd.type << "|" << cmd.name << "|" << cmd.message << "\n";
else
std::cout << "No match in '" << input << "'\n";
Live On Coliru
#include <iostream>
#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/home/x3.hpp>
#include <boost/spirit/home/x3/directive/confix.hpp>
namespace x3 = boost::spirit::x3;
namespace commands {
struct Command {
enum class Type { ask, message, notify } type;
std::string name;
std::string message;
friend std::ostream& operator<<(std::ostream& os, Type t) { return os << static_cast<int>(t); } // TODO
};
}
BOOST_FUSION_ADAPT_STRUCT(commands::Command, type, name, message)
namespace commands {
namespace grammar {
using namespace x3;
auto sentence = [](auto type, auto&& v, auto&& p) {
auto verb = lexeme [ no_case [ as_parser(v) ] ];
auto name = lexeme [ +graph ];
auto particle = lexeme [ no_case [ as_parser(p) ] ];
return attr(type) >> confix(verb, particle) [ name ];
};
using Type = Command::Type;
auto ask = sentence(Type::ask, "ask", "to") >> lexeme[+char_];
auto notify = sentence(Type::notify, "notify", "that") >> lexeme[+char_];
auto message = sentence(Type::message, "message", "that") >> lexeme[+char_];
auto command // = rule<struct command, Command> { }
= ask | notify | message;
auto parser = skip(space) [ command ];
}
}
int main() {
for (std::string const input : {
"Ask peter to call me",
"Notify Jenna that I am going to be away",
"Message home that I am running late",
})
{
commands::Command cmd;
if (parse(input.begin(), input.end(), commands::grammar::parser, cmd))
std::cout << "Matched: " << cmd.type << "|" << cmd.name << "|" << cmd.message << "\n";
else
std::cout << "No match in '" << input << "'\n";
}
}
Prints
Matched: 0|peter|call me
Matched: 2|Jenna|I am going to be away
Matched: 1|home|I am running late
¹ I'm no English linguist so I don't know whether that is the correct grammatical term :)

Answer 2

This code reads the command strings from the file "commands.txt", searches for the regular expressions and prints the parts whenever there is a match.
#include <iostream>
#include <fstream>
#include <string>
#include <boost/regex.hpp>
const int NumCmdParts = 4;
std::string CommandPartIds[] = {"Verb", "Name", "Preposition", "Content"};
int main(int argc, char *argv[])
{
std::ifstream ifs;
ifs.open ("commands.txt", std::ifstream::in);
if (!ifs.is_open()) {
std::cout << "Error opening file commands.txt" << std::endl;
exit(1);
}
std::string cmdStr;
// Pieces of regular expression pattern
// '(?<Verb>' : This is to name the capture group as 'Verb'
std::string VerbPat = "(?<Verb>(Ask)|(Notify|Message))";
std::string SeparatorPat = "\\s*";
std::string NamePat = "(?<Name>\\w+)";
// Conditional expression. if (Ask) (to) else (that)
std::string PrepositionPat = "(?<Preposition>(?(2)(to)|(that)))";
std::string ContentPat = "(?<Content>.*)";
// Put the pieces together to compose pattern
std::string TotalPat = VerbPat + SeparatorPat + NamePat + SeparatorPat
+ PrepositionPat + SeparatorPat + ContentPat;
boost::regex actions_re(TotalPat);
boost::smatch action_match;
while (getline(ifs, cmdStr)) {
bool IsMatch = boost::regex_search(cmdStr, action_match, actions_re);
if (IsMatch) {
for (int i=1; i <= NumCmdParts; i++) {
std::cout << CommandPartIds[i-1] << ": " << action_match[CommandPartIds[i-1]] << "\n";
}
}
}
ifs.close();
}