Add MTex.dirty_select

A more general way to select subparts by tex. Not as reliable as select_parts, but useful in many cases.

manimlib/mobject/svg/mtex_mobject.py

@@ -209,6 +209,39 @@ class MTex(StringMobject):
     ):
         return self.set_parts_color_by_dict(color_map)
 
+    @staticmethod
+    def n_symbols(tex) -> int:
+        """
+        This function attempts to estimate the number of symbols that
+        a given string of tex would produce.
+        
+        No guarantees this is accurate.
+        """
+        count_to_subtrs = [
+            (0, ["emph", "textbf", "big", "Big", "small", "Small"]),
+            (2, ["sqrt", "ne"]),
+            (6, ["underbrace"]),
+            # Replace all other \expressions (like "\pi") with a single character
+            # Deliberately put this last.
+            (1, ["[a-zA-Z]+"])
+        ]
+        for count, substrs in count_to_subtrs:
+            # Replace occurances of the given substrings with `count` characters
+            pattern = "|".join((R"\\" + s for s in substrs ))
+            tex = re.sub(pattern, "X" * count, tex)
+        # Ignore various control characters
+        return len(list(filter(lambda c: c not in "^{} \n\t_", tex)))
+
+    def dirty_select(self, substr: str) -> VGroup:
+        tex = self.get_tex()
+        result = []
+        for match in re.finditer(substr.replace("\\", R"\\"), tex):
+            index = match.start()
+            start = self.n_symbols(tex[:index])
+            end = start + self.n_symbols(substr)
+            result.append(self[start:end])
+        return VGroup(*result)
+
     def get_tex(self) -> str:
         return self.get_string()