Optimize performance and GC for string parsing in PDT.

Assets/Cryville/Common/Pdt/PdtInterpreter.cs

@@ -4,7 +4,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
-using System.Text.RegularExpressions;
+using System.Text;
 using CMath = System.Math;
 
 namespace Cryville.Common.Pdt {
@@ -64,6 +64,8 @@ namespace Cryville.Common.Pdt {
 		/// The current position in the string being parsed by the interpreter.
 		/// </summary>
 		public int Position { get; protected set; }
+
+		readonly StringBuilder _sb = new StringBuilder();
 #pragma warning disable IDE1006
 		/// <summary>
 		/// The character at the current position.
@@ -139,13 +141,15 @@ namespace Cryville.Common.Pdt {
 		/// <returns>A string.</returns>
 		/// <exception cref="IndexOutOfRangeException">The end of the source string is reached.</exception>
 		protected string GetString() {
-			int sp = Position;
-			do {
-				if (cc == '\\') Position++;
-				Position++;
-			} while (ct != 0x0100);
+			_sb.Clear();
 			Position++;
-			return Regex.Replace(Source.Substring(sp + 1, Position - sp - 2), @"\\(.)", "$1");
+			while (ct != CharCategory.StringDelimiter) {
+				if (cc == '\\') Position++;
+				_sb.Append(cc);
+				Position++;
+			}
+			Position++;
+			return _sb.ToString();
 		}
 		/// <summary>
 		/// Reads an expression.