Fix performance regression in 62119e5.

That commit caused unnecessary creation of string objects in
REGEX commands. Work around that by providing direct access
to the data buffer. Also use peek() instead of next() to
not construct unnecessary single-char string objects.

Addresses #8.

databuffer.hpp

@@ -30,6 +30,10 @@ class databuffer {
 	size_t line() const { return _line; }
 	size_t lpos() const { return _lpos; }
 
+	std::string::const_iterator curr()  const { return data.cbegin()+_pos; }
+	std::string::const_iterator begin() const { return data.cbegin(); }
+	std::string::const_iterator end()   const { return data.cend(); }
+
 	std::string next(size_t length=1) const
 	{
 		size_t end = std::min(size(),_pos+length);

libchecktestdata.cc

@@ -602,7 +602,7 @@ bool dotest(const test& t)
 		if ( gendata ) {
 			return (get_random(2) == 0);
 		} else {
-			return !data.eof() && t.args[0].val.find(data.next())!=string::npos;
+			return !data.eof() && t.args[0].val.find(data.peek())!=string::npos;
 		}
 	case 'U': return unique(t.args);
 	case 'A': return inarray(t.args[0],t.args[1]);
@@ -1010,8 +1010,9 @@ void checktoken(const command& cmd)
 			if ( toupper(data.readchar())!='E' ) error("exponent 'E' expected");
 			has_exp = true;
 			if ( data.peek()=='-' || data.peek()=='+' ) data.readchar();
-			while ( isdigit(data.peek()) ) data.readchar();
-			if ( !isdigit(data.peek(-1)) ) error("digit expected");
+			char c = '!';
+			while ( isdigit(data.peek()) ) c = data.readchar();
+			if ( !isdigit(c) ) error("digit expected");
 		}
 
 		if ( cmd.name()=="FLOATP" ) {
@@ -1055,8 +1056,8 @@ void checktoken(const command& cmd)
 		smatch res;
 		string matchstr;
 
-		string searchstr = data.next(data.size());
-		if ( !regex_search(searchstr,res,regexstr,regex_constants::match_continuous) ) {
+		if ( !regex_search(data.curr(),data.end(),res,regexstr,
+		                   regex_constants::match_continuous) ) {
 			error();
 		} else {
 			size_t match_len = res[0].second - res[0].first;