#include #include #include namespace grmr { // input stream struct struct input { std::string s; std::stack pos; int currPos = 0; input(std::string s) : s(s) { attempt(); } char read() { return s[currPos]; } char next() { return s[++currPos]; } // creates breakpoint at current pos void attempt() { pos.push(currPos); } // returns to the last breakpoint bool fail() { currPos = pos.top(); pos.pop(); return false; } // deletes last breakpoint bool success() { pos.pop(); return true; } }; // atom struct for matching one char template struct chr { static bool match(input& _input) { if(_input.read() == T) { _input.next(); return true; } return false; } }; // additional atom for range [a-z] template struct rng { static bool match(input& _input) { if(T1 <= _input.read() and _input.read() <= T2) { _input.next(); return true; } return false; } }; // matching both first and second template struct m_and { static bool match(input& _input) { _input.attempt(); if(!T1::match(_input)) return _input.fail(); if(T2::match(_input)) return _input.success(); return _input.fail(); } }; // matching either first or second (|) template struct m_or { static bool match(input& _input) { _input.attempt(); if(T1::match(_input)) return _input.success(); _input.fail(); _input.attempt(); if(T2::match(_input)) return _input.success(); return _input.fail(); } }; // matching if exsists or not ()? template struct q { static bool match(input& _input) { _input.attempt(); if(T::match(_input)) return _input.success(); return !_input.fail(); } }; // matching list of 0 and more ()* template struct ast { static bool match(input& _input) { _input.attempt(); while(T::match(_input)) { _input.success(); _input.attempt(); } return !_input.fail(); } }; // matching list of 1 or more ()+ template struct m_plus { static bool match(input& _input) { _input.attempt(); if(!T::match(_input)) return _input.fail(); _input.success(); _input.attempt(); while(T::match(_input)) { _input.success(); _input.attempt(); } return !_input.fail(); } }; // any lowercase letter // [a-z] typedef rng<'a','z'> low_ltr; // any uppercase letter // [A-Z] typedef rng<'A', 'Z'> up_ltr; // any letter // [a-zA-Z] typedef m_or ltr; // any letter + '_' sign typedef m_or> ltr_; // one digit // [0-9] typedef rng<'0', '9'> digit; // alpha-numeric // [a-zA-Z0-9] typedef m_or alnum; // alnum + '_' sign // [a-zA-Z_0-9] typedef m_or> alnum_; // std word definition // [a-zA-Z_][a-zA-Z_0-9]* typedef m_and> word; // space symbols [ \t\n] typedef m_or,m_or,chr<'\n'> >> space; // multispace [ \t\n]+ typedef m_plus mspace; } int main () { using namespace std; using namespace grmr; // \w\s+\w m_and> matcher; input in ("Hello World"); //match_res out; cout << in.s << endl; if (matcher.match(in)) cout << "matched!"; else cout << "nop"; return 0; }

how regex grammar works