regular expression generator from pattern (e.g. abba)

make_regex.cpp

#include <iostream>
#include <map>
#include <sstream>
#include <string>

std::string from_pattern(std::string pattern) {
  int i{0};
  std::map<char, int> m;
  std::ostringstream s;
  for (char c : pattern) {
    auto t = m.find(c);
    if (t != m.end()) {
      s << '\\' << t->second;
      continue;
    }
    t = m.begin();
    if (t != m.end()) {
      s << "(?!\\" << t->second;
      while (++t != m.end())
        s << "|\\" << t->second;
      s << ")";
    }
    s << "(.)";
    m.emplace(c, ++i);
  }
  return s.str();
}

int main() {
  std::string line;
  while (std::getline(std::cin, line))
    std::cout << line << " -> " << from_pattern(line) << std::endl;
  return 0;
}

Compile:

If you have a c++11 compliant compiler and make installed: CXXFLAGS='-std=c++11' make make_regex

If you have no idea, try: g++ -std=c++11 make_regex.cpp -o make_regex

Run:

./make_regex
abba
abba -> (.)(?!\1)(.)\2\1
abbbaaa
abbbaaa -> (.)(?!\1)(.)\2\2\1\1\1
abccba
abccba -> (.)(?!\1)(.)(?!\1|\2)(.)\3\2\1
abcdabcd
abcdabcd -> (.)(?!\1)(.)(?!\1|\2)(.)(?!\1|\2|\3)(.)\1\2\3\4
abcabcabc
abcabcabc -> (.)(?!\1)(.)(?!\1|\2)(.)\1\2\3\1\2\3
abbacca
abbacca -> (.)(?!\1)(.)\2\1(?!\1|\2)(.)\3\1
abbcccddddcccbba
abbcccddddcccbba -> (.)(?!\1)(.)\2(?!\1|\2)(.)\3\3(?!\1|\2|\3)(.)\4\4\4\3\3\3\2\2\1