nix-env: Add regular expression support in selectors

So you can now do things like:

  $ nix-env -qa '.*zip.*'
  $ nix-env -qa '.*(firefox|chromium).*'

src/libutil/regex.cc

@@ -0,0 +1,33 @@
+#include "regex.hh"
+#include "types.hh"
+
+namespace nix {
+
+Regex::Regex(const string & pattern)
+{
+    /* Patterns must match the entire string. */
+    int err = regcomp(&preg, ("^(" + pattern + ")$").c_str(), REG_NOSUB | REG_EXTENDED);
+    if (err) throw Error(format("compiling pattern ‘%1%’: %2%") % pattern % showError(err));
+}
+
+Regex::~Regex()
+{
+    regfree(&preg);
+}
+
+bool Regex::matches(const string & s)
+{
+    int err = regexec(&preg, s.c_str(), 0, 0, 0);
+    if (err == 0) return true;
+    else if (err == REG_NOMATCH) return false;
+    throw Error(format("matching string ‘%1%’: %2%") % s % showError(err));
+}
+
+string Regex::showError(int err)
+{
+    char buf[256];
+    regerror(err, &preg, buf, sizeof(buf));
+    return string(buf);
+}
+
+}