Pluggable fetchers

Flakes are now fetched using an extensible mechanism. Also lots of
other flake cleanups.

src/libstore/fetchers/regex.hh

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <regex>
+
+namespace nix::fetchers {
+
+// URI stuff.
+const static std::string pctEncoded = "%[0-9a-fA-F][0-9a-fA-F]";
+const static std::string schemeRegex = "[a-z+]+";
+const static std::string authorityRegex =
+    "(?:(?:[a-z])*@)?"
+    "[a-zA-Z0-9._~-]*";
+const static std::string segmentRegex = "[a-zA-Z0-9._~-]+";
+const static std::string pathRegex = "(?:/?" + segmentRegex + "(?:/" + segmentRegex + ")*|/?)";
+const static std::string pcharRegex =
+    "(?:[a-zA-Z0-9-._~!$&'()*+,;=:@ ]|" + pctEncoded + ")";
+const static std::string queryRegex = "(?:" + pcharRegex + "|[/?])*";
+
+// A Git ref (i.e. branch or tag name).
+const static std::string refRegexS = "[a-zA-Z0-9][a-zA-Z0-9_.-]*"; // FIXME: check
+extern std::regex refRegex;
+
+// A Git revision (a SHA-1 commit hash).
+const static std::string revRegexS = "[0-9a-fA-F]{40}";
+extern std::regex revRegex;
+
+// A ref or revision, or a ref followed by a revision.
+const static std::string refAndOrRevRegex = "(?:(" + revRegexS + ")|(?:(" + refRegexS + ")(?:/(" + revRegexS + "))?))";
+
+const static std::string flakeId = "[a-zA-Z][a-zA-Z0-9_-]*";
+
+}