diff --git a/core/io/dir_access.cpp b/core/io/dir_access.cpp
index 40c1a539588b..9749a2e3e734 100644
--- a/core/io/dir_access.cpp
+++ b/core/io/dir_access.cpp
@@ -159,11 +159,11 @@ Error DirAccess::make_dir_recursive(String p_dir) {
 		ERR_FAIL_COND_V(pos < 0, ERR_INVALID_PARAMETER);
 		pos = full_dir.find("/", pos + 1);
 		ERR_FAIL_COND_V(pos < 0, ERR_INVALID_PARAMETER);
-		base = full_dir.substr(0, pos + 1);
+		base = full_dir.left(pos + 1);
 	} else if (full_dir.begins_with("/")) {
 		base = "/";
 	} else if (full_dir.contains(":/")) {
-		base = full_dir.substr(0, full_dir.find(":/") + 2);
+		base = full_dir.left(full_dir.find(":/") + 2);
 	} else {
 		ERR_FAIL_V(ERR_INVALID_PARAMETER);
 	}
diff --git a/core/io/file_access.cpp b/core/io/file_access.cpp
index 6026dbf8962f..a6f2288a977f 100644
--- a/core/io/file_access.cpp
+++ b/core/io/file_access.cpp
@@ -417,7 +417,7 @@ Vector<String> FileAccess::get_csv_line(const String &p_delim) const {
 	} while (qc % 2);
 
 	// Remove the extraneous newline we've added above.
-	line = line.substr(0, line.length() - 1);
+	line = line.left(-1);
 
 	Vector<String> strings;
 
diff --git a/core/io/file_access_compressed.cpp b/core/io/file_access_compressed.cpp
index 0f00bd292cdd..755d293f9c6c 100644
--- a/core/io/file_access_compressed.cpp
+++ b/core/io/file_access_compressed.cpp
@@ -34,7 +34,7 @@
 
 void FileAccessCompressed::configure(const String &p_magic, Compression::Mode p_mode, uint32_t p_block_size) {
 	magic = p_magic.ascii().get_data();
-	magic = (magic + "    ").substr(0, 4);
+	magic = magic.rpad(4, " ");
 
 	cmode = p_mode;
 	block_size = p_block_size;
diff --git a/core/io/http_client.cpp b/core/io/http_client.cpp
index 09505ea05da5..0a81b5a99b56 100644
--- a/core/io/http_client.cpp
+++ b/core/io/http_client.cpp
@@ -94,7 +94,7 @@ String HTTPClient::query_string_from_dict(const Dictionary &p_dict) {
 			}
 		}
 	}
-	return query.substr(1);
+	return query.right(-1);
 }
 
 Error HTTPClient::verify_headers(const Vector<String> &p_headers) {
@@ -117,8 +117,8 @@ Dictionary HTTPClient::_get_response_headers_as_dictionary() {
 		if (sp == -1) {
 			continue;
 		}
-		String key = s.substr(0, sp).strip_edges();
-		String value = s.substr(sp + 1, s.length()).strip_edges();
+		String key = s.left(sp).strip_edges();
+		String value = s.substr(sp + 1).strip_edges();
 		ret[key] = value;
 	}
 
diff --git a/core/string/node_path.cpp b/core/string/node_path.cpp
index 32e4564c5e1a..212412a75107 100644
--- a/core/string/node_path.cpp
+++ b/core/string/node_path.cpp
@@ -393,7 +393,7 @@ NodePath::NodePath(const String &p_path) {
 			}
 		}
 
-		path = path.substr(0, subpath_pos);
+		path = path.left(subpath_pos);
 	}
 
 	for (int i = (int)absolute; i < path.length(); i++) {
diff --git a/core/string/translation.cpp b/core/string/translation.cpp
index a443ed308d62..130401f6e149 100644
--- a/core/string/translation.cpp
+++ b/core/string/translation.cpp
@@ -1018,9 +1018,9 @@ void TranslationServer::_bind_methods() {
 
 void TranslationServer::load_translations() {
 	_load_translations("internationalization/locale/translations"); //all
-	_load_translations("internationalization/locale/translations_" + locale.substr(0, 2));
+	_load_translations("internationalization/locale/translations_" + locale.left(2));
 
-	if (locale.substr(0, 2) != locale) {
+	if (locale.left(2) != locale) {
 		_load_translations("internationalization/locale/translations_" + locale);
 	}
 }
diff --git a/core/string/translation_po.cpp b/core/string/translation_po.cpp
index 6b1595174ad5..5f2d8ba0a39a 100644
--- a/core/string/translation_po.cpp
+++ b/core/string/translation_po.cpp
@@ -172,10 +172,10 @@ void TranslationPO::_cache_plural_tests(const String &p_plural_rule) {
 		return;
 	}
 
-	String equi_test = p_plural_rule.substr(0, first_ques_mark).strip_edges();
+	String equi_test = p_plural_rule.left(first_ques_mark).strip_edges();
 	equi_tests.push_back(equi_test);
 
-	String after_colon = p_plural_rule.substr(p_plural_rule.find(":") + 1, p_plural_rule.length());
+	String after_colon = p_plural_rule.substr(p_plural_rule.find(":") + 1);
 	_cache_plural_tests(after_colon);
 }
 
diff --git a/core/string/ustring.cpp b/core/string/ustring.cpp
index be829502efc1..941dd9112018 100644
--- a/core/string/ustring.cpp
+++ b/core/string/ustring.cpp
@@ -231,20 +231,20 @@ Error String::parse_url(String &r_scheme, String &r_host, int &r_port, String &r
 	int pos = base.find("://");
 	// Scheme
 	if (pos != -1) {
-		r_scheme = base.substr(0, pos + 3).to_lower();
-		base = base.substr(pos + 3, base.length() - pos - 3);
+		r_scheme = base.left(pos + 3).to_lower();
+		base = base.substr(pos + 3);
 	}
 	pos = base.find("/");
 	// Path
 	if (pos != -1) {
-		r_path = base.substr(pos, base.length() - pos);
-		base = base.substr(0, pos);
+		r_path = base.right(-pos);
+		base = base.left(pos);
 	}
 	// Host
 	pos = base.find("@");
 	if (pos != -1) {
-		// Strip credentials
-		base = base.substr(pos + 1, base.length() - pos - 1);
+		// Strip credentials.
+		base = base.substr(pos + 1);
 	}
 	if (base.begins_with("[")) {
 		// Literal IPv6
@@ -253,7 +253,7 @@ Error String::parse_url(String &r_scheme, String &r_host, int &r_port, String &r
 			return ERR_INVALID_PARAMETER;
 		}
 		r_host = base.substr(1, pos - 1);
-		base = base.substr(pos + 1, base.length() - pos - 1);
+		base = base.substr(pos + 1);
 	} else {
 		// Anything else
 		if (base.get_slice_count(":") > 2) {
@@ -264,8 +264,8 @@ Error String::parse_url(String &r_scheme, String &r_host, int &r_port, String &r
 			r_host = base;
 			base = "";
 		} else {
-			r_host = base.substr(0, pos);
-			base = base.substr(pos, base.length() - pos);
+			r_host = base.left(pos);
+			base = base.right(-pos);
 		}
 	}
 	if (r_host.is_empty()) {
@@ -1297,9 +1297,9 @@ Vector<String> String::rsplit(const String &p_splitter, bool p_allow_empty, int
 
 	while (true) {
 		if (remaining_len < p_splitter.length() || (p_maxsplit > 0 && p_maxsplit == ret.size())) {
-			// no room for another splitter or hit max splits, push what's left and we're done
+			// No room for another splitter or hit max splits, push what's left and we're done.
 			if (p_allow_empty || remaining_len > 0) {
-				ret.push_back(substr(0, remaining_len));
+				ret.push_back(left(remaining_len));
 			}
 			break;
 		}
@@ -1315,8 +1315,8 @@ Vector<String> String::rsplit(const String &p_splitter, bool p_allow_empty, int
 		}
 
 		if (left_edge < 0) {
-			// no more splitters, we're done
-			ret.push_back(substr(0, remaining_len));
+			// No more splitters, we're done.
+			ret.push_back(left(remaining_len));
 			break;
 		}
 
@@ -2389,7 +2389,7 @@ int64_t String::to_int(const char *p_str, int p_len) {
 		char c = p_str[i];
 		if (is_digit(c)) {
 			bool overflow = (integer > INT64_MAX / 10) || (integer == INT64_MAX / 10 && ((sign == 1 && c > '7') || (sign == -1 && c > '8')));
-			ERR_FAIL_COND_V_MSG(overflow, sign == 1 ? INT64_MAX : INT64_MIN, "Cannot represent " + String(p_str).substr(0, to) + " as a 64-bit signed integer, since the value is " + (sign == 1 ? "too large." : "too small."));
+			ERR_FAIL_COND_V_MSG(overflow, sign == 1 ? INT64_MAX : INT64_MIN, "Cannot represent " + String(p_str).left(to) + " as a 64-bit signed integer, since the value is " + (sign == 1 ? "too large." : "too small."));
 			integer *= 10;
 			integer += c - '0';
 
@@ -2420,7 +2420,7 @@ int64_t String::to_int(const wchar_t *p_str, int p_len) {
 		wchar_t c = p_str[i];
 		if (is_digit(c)) {
 			bool overflow = (integer > INT64_MAX / 10) || (integer == INT64_MAX / 10 && ((sign == 1 && c > '7') || (sign == -1 && c > '8')));
-			ERR_FAIL_COND_V_MSG(overflow, sign == 1 ? INT64_MAX : INT64_MIN, "Cannot represent " + String(p_str).substr(0, to) + " as a 64-bit signed integer, since the value is " + (sign == 1 ? "too large." : "too small."));
+			ERR_FAIL_COND_V_MSG(overflow, sign == 1 ? INT64_MAX : INT64_MIN, "Cannot represent " + String(p_str).left(to) + " as a 64-bit signed integer, since the value is " + (sign == 1 ? "too large." : "too small."));
 			integer *= 10;
 			integer += c - '0';
 
@@ -2949,21 +2949,15 @@ String String::insert(int p_at_pos, const String &p_string) const {
 		return *this;
 	}
 
-	if (p_at_pos > length()) {
-		p_at_pos = length();
+	if (p_at_pos >= length()) {
+		return *this + p_string;
 	}
 
-	String pre;
-	if (p_at_pos > 0) {
-		pre = substr(0, p_at_pos);
+	if (p_at_pos == 0) {
+		return p_string + *this;
 	}
 
-	String post;
-	if (p_at_pos < length()) {
-		post = substr(p_at_pos, length() - p_at_pos);
-	}
-
-	return pre + p_string + post;
+	return left(p_at_pos) + p_string + right(-p_at_pos);
 }
 
 String String::erase(int p_pos, int p_chars) const {
@@ -3579,7 +3573,7 @@ String String::replace(const String &p_key, const String &p_with) const {
 		return *this;
 	}
 
-	new_string += substr(search_from, length() - search_from);
+	new_string += right(-search_from);
 
 	return new_string;
 }
@@ -3603,7 +3597,7 @@ String String::replace(const char *p_key, const char *p_with) const {
 		return *this;
 	}
 
-	new_string += substr(search_from, length() - search_from);
+	new_string += right(-search_from);
 
 	return new_string;
 }
@@ -3611,7 +3605,7 @@ String String::replace(const char *p_key, const char *p_with) const {
 String String::replace_first(const String &p_key, const String &p_with) const {
 	int pos = find(p_key);
 	if (pos >= 0) {
-		return substr(0, pos) + p_with + substr(pos + p_key.length(), length());
+		return left(pos) + p_with + substr(pos + p_key.length(), length());
 	}
 
 	return *this;
@@ -3632,7 +3626,7 @@ String String::replacen(const String &p_key, const String &p_with) const {
 		return *this;
 	}
 
-	new_string += substr(search_from, length() - search_from);
+	new_string += right(-search_from);
 	return new_string;
 }
 
@@ -3682,7 +3676,7 @@ String String::reverse() const {
 
 String String::left(int p_len) const {
 	if (p_len < 0) {
-		p_len = length() + p_len;
+		p_len += length();
 	}
 
 	if (p_len <= 0) {
@@ -3700,7 +3694,7 @@ String String::left(int p_len) const {
 
 String String::right(int p_len) const {
 	if (p_len < 0) {
-		p_len = length() + p_len;
+		p_len += length();
 	}
 
 	if (p_len <= 0) {
@@ -3737,7 +3731,7 @@ String String::indent(const String &p_prefix) const {
 		}
 	}
 	if (line_start != length()) {
-		new_string += p_prefix + substr(line_start);
+		new_string += p_prefix + right(-line_start);
 	}
 	return new_string;
 }
@@ -3779,7 +3773,7 @@ String String::dedent() const {
 	}
 
 	if (has_text) {
-		new_string += substr(indent_stop, length() - indent_stop);
+		new_string += right(-indent_stop);
 	}
 
 	return new_string;
@@ -3856,11 +3850,7 @@ String String::rstrip(const String &p_chars) const {
 		}
 	}
 
-	if (end == len - 1) {
-		return *this;
-	}
-
-	return substr(0, end + 1);
+	return left(end + 1);
 }
 
 bool String::is_network_share_path() const {
@@ -3884,19 +3874,19 @@ String String::simplify_path() const {
 		}
 		if (only_chars) {
 			found = true;
-			drive = s.substr(0, p + 3);
+			drive = s.left(p + 3);
 			s = s.substr(p + 3);
 		}
 	}
 	if (!found) {
 		if (is_network_share_path()) {
 			// Network path, beginning with // or \\.
-			drive = s.substr(0, 2);
-			s = s.substr(2);
+			drive = s.left(2);
+			s = s.right(-2);
 		} else if (s.begins_with("/") || s.begins_with("\\")) {
 			// Absolute path.
-			drive = s.substr(0, 1);
-			s = s.substr(1);
+			drive = s.left(1);
+			s = s.right(-1);
 		} else {
 			// Windows-style drive path, like C:/ or C:\.
 			p = s.find(":/");
@@ -3904,7 +3894,7 @@ String String::simplify_path() const {
 				p = s.find(":\\");
 			}
 			if (p != -1 && p < s.find("/")) {
-				drive = s.substr(0, p + 2);
+				drive = s.left(p + 2);
 				s = s.substr(p + 2);
 			}
 		}
@@ -4302,12 +4292,12 @@ String String::pad_decimals(int p_digits) const {
 		c = s.length() - 1;
 	} else {
 		if (p_digits <= 0) {
-			return s.substr(0, c);
+			return s.left(c);
 		}
 	}
 
 	if (s.length() - (c + 1) > p_digits) {
-		return s.substr(0, c + p_digits + 1);
+		return s.left(c + p_digits + 1);
 	} else {
 		int zeros_to_add = p_digits - s.length() + (c + 1);
 		return s + String("0").repeat(zeros_to_add);
@@ -4344,7 +4334,7 @@ String String::pad_zeros(int p_digits) const {
 String String::trim_prefix(const String &p_prefix) const {
 	String s = *this;
 	if (s.begins_with(p_prefix)) {
-		return s.substr(p_prefix.length(), s.length() - p_prefix.length());
+		return s.right(-p_prefix.length());
 	}
 	return s;
 }
@@ -4352,7 +4342,7 @@ String String::trim_prefix(const String &p_prefix) const {
 String String::trim_suffix(const String &p_suffix) const {
 	String s = *this;
 	if (s.ends_with(p_suffix)) {
-		return s.substr(0, s.length() - p_suffix.length());
+		return s.left(-p_suffix.length());
 	}
 	return s;
 }
@@ -4371,7 +4361,7 @@ bool String::is_valid_int() const {
 
 	for (int i = from; i < len; i++) {
 		if (!is_digit(operator[](i))) {
-			return false; // no start with number plz
+			return false; // I hereby desire to not start with a number.
 		}
 	}
 
@@ -4495,11 +4485,11 @@ String String::path_to(const String &p_path) const {
 		dst = dst.substr(dst_begin.length(), dst.length());
 	}
 
-	//remove leading and trailing slash and split
+	// Remove leading and trailing slash and split.
 	Vector<String> src_dirs = src.substr(1, src.length() - 2).split("/");
 	Vector<String> dst_dirs = dst.substr(1, dst.length() - 2).split("/");
 
-	//find common parent
+	// Find common parent.
 	int common_parent = 0;
 
 	while (true) {
@@ -4660,8 +4650,8 @@ String String::get_base_dir() const {
 	String rs;
 	String base;
 	if (end != 0) {
-		rs = substr(end, length());
-		base = substr(0, end);
+		rs = right(-end);
+		base = left(end);
 	} else {
 		rs = *this;
 	}
@@ -4671,7 +4661,7 @@ String String::get_base_dir() const {
 		return base;
 	}
 
-	return base + rs.substr(0, sep);
+	return base + rs.left(sep);
 }
 
 String String::get_file() const {
@@ -4680,7 +4670,7 @@ String String::get_file() const {
 		return *this;
 	}
 
-	return substr(sep + 1, length());
+	return substr(sep + 1);
 }
 
 String String::get_extension() const {
@@ -4689,7 +4679,7 @@ String String::get_extension() const {
 		return "";
 	}
 
-	return substr(pos + 1, length());
+	return substr(pos + 1);
 }
 
 String String::path_join(const String &p_file) const {
@@ -4788,7 +4778,7 @@ String String::get_basename() const {
 		return *this;
 	}
 
-	return substr(0, pos);
+	return left(pos);
 }
 
 String itos(int64_t p_val) {
@@ -5201,7 +5191,9 @@ String String::unquote() const {
 		return *this;
 	}
 
-	return substr(1, length() - 2);
+	String s;
+	s.copy_from_unchecked(&get_data()[1], length() - 2);
+	return s;
 }
 
 Vector<uint8_t> String::to_ascii_buffer() const {
diff --git a/editor/code_editor.cpp b/editor/code_editor.cpp
index 9a107669001a..6c8f89c4bc1e 100644
--- a/editor/code_editor.cpp
+++ b/editor/code_editor.cpp
@@ -1118,7 +1118,7 @@ void CodeTextEditor::trim_trailing_whitespace() {
 					break;
 				}
 			}
-			text_editor->set_line(i, line.substr(0, end));
+			text_editor->set_line(i, line.left(end));
 		}
 	}
 
diff --git a/editor/editor_help.cpp b/editor/editor_help.cpp
index 10ab733c2bd5..bd39e94571a1 100644
--- a/editor/editor_help.cpp
+++ b/editor/editor_help.cpp
@@ -2130,7 +2130,7 @@ static void _add_text_to_rt(const String &p_bbcode, RichTextLabel *p_rt, Control
 
 		} else if (tag.begins_with("method ") || tag.begins_with("constructor ") || tag.begins_with("operator ") || tag.begins_with("member ") || tag.begins_with("signal ") || tag.begins_with("enum ") || tag.begins_with("constant ") || tag.begins_with("annotation ") || tag.begins_with("theme_item ")) {
 			const int tag_end = tag.find(" ");
-			const String link_tag = tag.substr(0, tag_end);
+			const String link_tag = tag.left(tag_end);
 			const String link_target = tag.substr(tag_end + 1, tag.length()).lstrip(" ");
 
 			// Use monospace font to make clickable references
@@ -2299,7 +2299,7 @@ static void _add_text_to_rt(const String &p_bbcode, RichTextLabel *p_rt, Control
 					const String &expr = subtags[i];
 					int value_pos = expr.find("=");
 					if (value_pos > -1) {
-						bbcode_options[expr.substr(0, value_pos)] = expr.substr(value_pos + 1).unquote();
+						bbcode_options[expr.left(value_pos)] = expr.substr(value_pos + 1).unquote();
 					}
 				}
 				HashMap<String, String>::Iterator width_option = bbcode_options.find("width");
diff --git a/editor/editor_settings.cpp b/editor/editor_settings.cpp
index 1eaeee97a578..9612df558c76 100644
--- a/editor/editor_settings.cpp
+++ b/editor/editor_settings.cpp
@@ -1367,7 +1367,7 @@ bool EditorSettings::save_text_editor_theme_as(String p_file) {
 	if (_save_text_editor_theme(p_file)) {
 		// switch to theme is saved in the theme directory
 		list_text_editor_themes();
-		String theme_name = p_file.substr(0, p_file.length() - 4).get_file();
+		String theme_name = p_file.left(-4).get_file();
 
 		if (p_file.get_base_dir() == EditorPaths::get_singleton()->get_text_editor_themes_dir()) {
 			_initial_set("text_editor/theme/color_theme", theme_name);
diff --git a/modules/mono/utils/path_utils.cpp b/modules/mono/utils/path_utils.cpp
index aa975346756c..6f2346ba66dc 100644
--- a/modules/mono/utils/path_utils.cpp
+++ b/modules/mono/utils/path_utils.cpp
@@ -213,7 +213,7 @@ String relative_to_impl(const String &p_path, const String &p_relative_to) {
 String get_drive_letter(const String &p_norm_path) {
 	int idx = p_norm_path.find(":/");
 	if (idx != -1 && idx < p_norm_path.find("/")) {
-		return p_norm_path.substr(0, idx + 1);
+		return p_norm_path.left(idx + 1);
 	}
 	return String();
 }
diff --git a/scene/gui/code_edit.cpp b/scene/gui/code_edit.cpp
index 291d578add17..d6f749404e76 100644
--- a/scene/gui/code_edit.cpp
+++ b/scene/gui/code_edit.cpp
@@ -145,7 +145,7 @@ void CodeEdit::_notification(int p_what) {
 
 					for (int j = 0; j < code_completion_options[l].matches.size(); j++) {
 						Pair<int, int> match_segment = code_completion_options[l].matches[j];
-						int match_offset = theme_cache.font->get_string_size(code_completion_options[l].display.substr(0, match_segment.first), HORIZONTAL_ALIGNMENT_LEFT, -1, theme_cache.font_size).width;
+						int match_offset = theme_cache.font->get_string_size(code_completion_options[l].display.left(match_segment.first), HORIZONTAL_ALIGNMENT_LEFT, -1, theme_cache.font_size).width;
 						int match_len = theme_cache.font->get_string_size(code_completion_options[l].display.substr(match_segment.first, match_segment.second), HORIZONTAL_ALIGNMENT_LEFT, -1, theme_cache.font_size).width;
 
 						draw_rect(Rect2(match_pos + Point2(match_offset, 0), Size2(match_len, row_height)), theme_cache.code_completion_existing_color);
@@ -176,7 +176,7 @@ void CodeEdit::_notification(int p_what) {
 				}
 				Size2 minsize = theme_cache.code_hint_style->get_minimum_size() + Size2(max_width, line_count * font_height + (theme_cache.line_spacing * line_count - 1));
 
-				int offset = theme_cache.font->get_string_size(code_hint_lines[0].substr(0, code_hint_lines[0].find(String::chr(0xFFFF))), HORIZONTAL_ALIGNMENT_LEFT, -1, theme_cache.font_size).x;
+				int offset = theme_cache.font->get_string_size(code_hint_lines[0].get_slice(String::chr(0xFFFF), 0), HORIZONTAL_ALIGNMENT_LEFT, -1, theme_cache.font_size).x;
 				if (code_hint_xpos == -0xFFFF) {
 					code_hint_xpos = get_caret_draw_pos().x - offset;
 				}
@@ -196,8 +196,8 @@ void CodeEdit::_notification(int p_what) {
 					int begin = 0;
 					int end = 0;
 					if (line.contains(String::chr(0xFFFF))) {
-						begin = theme_cache.font->get_string_size(line.substr(0, line.find(String::chr(0xFFFF))), HORIZONTAL_ALIGNMENT_LEFT, -1, theme_cache.font_size).x;
-						end = theme_cache.font->get_string_size(line.substr(0, line.rfind(String::chr(0xFFFF))), HORIZONTAL_ALIGNMENT_LEFT, -1, theme_cache.font_size).x;
+						begin = theme_cache.font->get_string_size(line.left(line.find(String::chr(0xFFFF))), HORIZONTAL_ALIGNMENT_LEFT, -1, theme_cache.font_size).x;
+						end = theme_cache.font->get_string_size(line.left(line.rfind(String::chr(0xFFFF))), HORIZONTAL_ALIGNMENT_LEFT, -1, theme_cache.font_size).x;
 					}
 
 					Point2 round_ofs = hint_ofs + theme_cache.code_hint_style->get_offset() + Vector2(0, theme_cache.font->get_ascent(theme_cache.font_size) + font_height * i + yofs);
@@ -914,7 +914,7 @@ void CodeEdit::unindent_lines() {
 			String line_text = get_line(i);
 
 			if (line_text.begins_with("\t")) {
-				line_text = line_text.substr(1, line_text.length());
+				line_text = line_text.right(-1);
 
 				set_line(i, line_text);
 				removed_characters = 1;
@@ -929,7 +929,7 @@ void CodeEdit::unindent_lines() {
 				// Here we remove only enough spaces to align text to nearest full multiple of indentation_size.
 				// In case where selection begins at the start of indentation_size multiple we remove whole indentation level.
 				int spaces_to_remove = _calculate_spaces_till_next_left_indent(get_first_non_whitespace_column(i));
-				line_text = line_text.substr(spaces_to_remove, line_text.length());
+				line_text = line_text.substr(spaces_to_remove);
 
 				set_line(i, line_text);
 				removed_characters = spaces_to_remove;
@@ -2067,10 +2067,10 @@ String CodeEdit::get_text_for_code_completion() const {
 		String line = get_line(i);
 
 		if (i == get_caret_line()) {
-			completion_text += line.substr(0, get_caret_column());
+			completion_text += line.left(get_caret_column());
 			/* Not unicode, represents the caret. */
 			completion_text += String::chr(0xFFFF);
-			completion_text += line.substr(get_caret_column(), line.size());
+			completion_text += line.substr(get_caret_column());
 		} else {
 			completion_text += line;
 		}
@@ -2262,7 +2262,7 @@ void CodeEdit::confirm_code_completion(bool p_replace) {
 			set_caret_column(get_caret_column(i) - code_completion_base.length(), false, i);
 
 			// Merge with text.
-			insert_text_at_caret(insert_text.substr(0, code_completion_base.length()), i);
+			insert_text_at_caret(insert_text.left(code_completion_base.length()), i);
 			set_caret_column(caret_col, false, i);
 			insert_text_at_caret(insert_text.substr(matching_chars), i);
 		}
@@ -2364,10 +2364,10 @@ String CodeEdit::get_text_with_cursor_char(int p_line, int p_column) const {
 	for (int i = 0; i < text_size; i++) {
 		String line_text = get_line(i);
 		if (i == p_line && p_column >= 0 && p_column <= line_text.size()) {
-			result += line_text.substr(0, p_column);
+			result += line_text.left(p_column);
 			/* Not unicode, represents the cursor. */
 			result += String::chr(0xFFFF);
-			result += line_text.substr(p_column, line_text.size());
+			result += line_text.substr(p_column);
 		} else {
 			result += line_text;
 		}
diff --git a/scene/gui/file_dialog.cpp b/scene/gui/file_dialog.cpp
index d721ee3ec3e8..d54956758714 100644
--- a/scene/gui/file_dialog.cpp
+++ b/scene/gui/file_dialog.cpp
@@ -800,8 +800,8 @@ void FileDialog::set_current_path(const String &p_path) {
 	if (pos == -1) {
 		set_current_file(p_path);
 	} else {
-		String path_dir = p_path.substr(0, pos);
-		String path_file = p_path.substr(pos + 1, p_path.length());
+		String path_dir = p_path.left(pos);
+		String path_file = p_path.substr(pos + 1);
 		set_current_dir(path_dir);
 		set_current_file(path_file);
 	}
diff --git a/scene/gui/label.cpp b/scene/gui/label.cpp
index 2fbd29b04801..692c551fccbf 100644
--- a/scene/gui/label.cpp
+++ b/scene/gui/label.cpp
@@ -119,7 +119,7 @@ void Label::_shape() {
 		ERR_FAIL_COND(font.is_null());
 		String txt = (uppercase) ? TS->string_to_upper(xl_text, language) : xl_text;
 		if (visible_chars >= 0 && visible_chars_behavior == TextServer::VC_CHARS_BEFORE_SHAPING) {
-			txt = txt.substr(0, visible_chars);
+			txt = txt.left(visible_chars);
 		}
 		if (dirty) {
 			TS->shaped_text_add_string(text_rid, txt, font->get_rids(), font_size, font->get_opentype_features(), language);
diff --git a/scene/gui/line_edit.cpp b/scene/gui/line_edit.cpp
index 5ed1a9d5e3ab..fc3dd7ec740c 100644
--- a/scene/gui/line_edit.cpp
+++ b/scene/gui/line_edit.cpp
@@ -141,7 +141,7 @@ void LineEdit::_backspace(bool p_word, bool p_all_to_left) {
 
 	if (p_all_to_left) {
 		deselect();
-		text = text.substr(0, caret_column);
+		text = text.left(caret_column);
 		_text_changed();
 		return;
 	}
@@ -177,7 +177,7 @@ void LineEdit::_delete(bool p_word, bool p_all_to_right) {
 
 	if (p_all_to_right) {
 		deselect();
-		text = text.substr(caret_column, text.length() - caret_column);
+		text = text.substr(caret_column);
 		_shape();
 		set_caret_column(0);
 		_text_changed();
@@ -1739,11 +1739,11 @@ void LineEdit::insert_text_at_caret(String p_text) {
 		int available_chars = max_length - text.length();
 		if (p_text.length() > available_chars) {
 			emit_signal(SNAME("text_change_rejected"), p_text.substr(available_chars));
-			p_text = p_text.substr(0, available_chars);
+			p_text = p_text.left(available_chars);
 		}
 	}
-	String pre = text.substr(0, caret_column);
-	String post = text.substr(caret_column, text.length() - caret_column);
+	String pre = text.left(caret_column);
+	String post = text.substr(caret_column);
 	text = pre + p_text + post;
 	_shape();
 	TextServer::Direction dir = TS->shaped_text_get_dominant_direction_in_range(text_rid, caret_column, caret_column + p_text.length());
@@ -2295,7 +2295,7 @@ void LineEdit::_shape() {
 		t = secret.repeat(text.length() + ime_text.length());
 	} else {
 		if (ime_text.length() > 0) {
-			t = text.substr(0, caret_column) + ime_text + text.substr(caret_column, text.length());
+			t = text.left(caret_column) + ime_text + text.substr(caret_column);
 		} else {
 			t = text;
 		}
diff --git a/scene/gui/rich_text_label.cpp b/scene/gui/rich_text_label.cpp
index dc96f8a59484..9807d9ffa91c 100644
--- a/scene/gui/rich_text_label.cpp
+++ b/scene/gui/rich_text_label.cpp
@@ -574,7 +574,7 @@ float RichTextLabel::_shape_line(ItemFrame *p_frame, int p_line, const Ref<Font>
 				String lang = _find_language(it);
 				String tx = t->text;
 				if (visible_chars_behavior == TextServer::VC_CHARS_BEFORE_SHAPING && visible_characters >= 0 && remaining_characters >= 0) {
-					tx = tx.substr(0, remaining_characters);
+					tx = tx.left(remaining_characters);
 				}
 				remaining_characters -= tx.length();
 
@@ -4041,7 +4041,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 
 		if (brk_end == -1) {
 			//no close, add the rest
-			txt += p_bbcode.substr(brk_pos, p_bbcode.length() - brk_pos);
+			txt += p_bbcode.substr(brk_pos);
 			add_text(txt);
 			break;
 		}
@@ -4059,7 +4059,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 				const String &expr = split_tag_block[i];
 				int value_pos = expr.find("=");
 				if (value_pos > -1) {
-					bbcode_options[expr.substr(0, value_pos)] = expr.substr(value_pos + 1).unquote();
+					bbcode_options[expr.left(value_pos)] = expr.substr(value_pos + 1).unquote();
 				}
 			}
 		} else {
@@ -4071,11 +4071,11 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 		int main_value_pos = bbcode_name.find("=");
 		if (main_value_pos > -1) {
 			bbcode_value = bbcode_name.substr(main_value_pos + 1);
-			bbcode_name = bbcode_name.substr(0, main_value_pos);
+			bbcode_name = bbcode_name.left(main_value_pos);
 		}
 
 		if (tag.begins_with("/") && tag_stack.size()) {
-			bool tag_ok = tag_stack.size() && tag_stack.front()->get() == tag.substr(1, tag.length());
+			bool tag_ok = tag_stack.size() && tag_stack.front()->get() == tag.right(-1);
 
 			if (tag_stack.front()->get() == "b") {
 				in_bold = false;
@@ -4403,7 +4403,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 			pos = brk_end + 1;
 			tag_stack.push_front(tag);
 		} else if (tag.begins_with("lang=")) {
-			String lang = tag.substr(5, tag.length()).unquote();
+			String lang = tag.right(-5).unquote();
 			push_language(lang);
 			pos = brk_end + 1;
 			tag_stack.push_front("lang");
@@ -4412,7 +4412,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 			pos = brk_end + 1;
 			tag_stack.push_front("p");
 		} else if (tag.begins_with("p ")) {
-			Vector<String> subtag = tag.substr(2, tag.length()).split(" ");
+			Vector<String> subtag = tag.right(-2).split(" ");
 			_normalize_subtags(subtag);
 
 			HorizontalAlignment alignment = HORIZONTAL_ALIGNMENT_LEFT;
@@ -4504,17 +4504,17 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 			tag_stack.push_front(tag);
 
 		} else if (tag.begins_with("url=")) {
-			String url = tag.substr(4, tag.length()).unquote();
+			String url = tag.right(-4).unquote();
 			push_meta(url);
 			pos = brk_end + 1;
 			tag_stack.push_front("url");
 		} else if (tag.begins_with("hint=")) {
-			String description = tag.substr(5, tag.length()).unquote();
+			String description = tag.right(-5).unquote();
 			push_hint(description);
 			pos = brk_end + 1;
 			tag_stack.push_front("hint");
 		} else if (tag.begins_with("dropcap")) {
-			Vector<String> subtag = tag.substr(5, tag.length()).split(" ");
+			Vector<String> subtag = tag.substr(-5).split(" ");
 			_normalize_subtags(subtag);
 
 			int fs = theme_cache.normal_font_size * 3;
@@ -4570,7 +4570,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 		} else if (tag.begins_with("img")) {
 			int alignment = INLINE_ALIGNMENT_CENTER;
 			if (tag.begins_with("img=")) {
-				Vector<String> subtag = tag.substr(4, tag.length()).split(",");
+				Vector<String> subtag = tag.right(-4).split(",");
 				_normalize_subtags(subtag);
 
 				if (subtag.size() > 1) {
@@ -4638,7 +4638,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 					if (sep == -1) {
 						width = bbcode_value.to_int();
 					} else {
-						width = bbcode_value.substr(0, sep).to_int();
+						width = bbcode_value.left(sep).to_int();
 						height = bbcode_value.substr(sep + 1).to_int();
 					}
 				} else {
@@ -4675,21 +4675,21 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 			pos = end;
 			tag_stack.push_front(bbcode_name);
 		} else if (tag.begins_with("color=")) {
-			String color_str = tag.substr(6, tag.length()).unquote();
+			String color_str = tag.right(-6).unquote();
 			Color color = Color::from_string(color_str, theme_cache.default_color);
 			push_color(color);
 			pos = brk_end + 1;
 			tag_stack.push_front("color");
 
 		} else if (tag.begins_with("outline_color=")) {
-			String color_str = tag.substr(14, tag.length()).unquote();
+			String color_str = tag.right(-14).unquote();
 			Color color = Color::from_string(color_str, theme_cache.default_color);
 			push_outline_color(color);
 			pos = brk_end + 1;
 			tag_stack.push_front("outline_color");
 
 		} else if (tag.begins_with("font_size=")) {
-			int fnt_size = tag.substr(10, tag.length()).to_int();
+			int fnt_size = tag.right(-10).to_int();
 			push_font_size(fnt_size);
 			pos = brk_end + 1;
 			tag_stack.push_front("font_size");
@@ -4736,10 +4736,10 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 				}
 			}
 			pos = brk_end + 1;
-			tag_stack.push_front(tag.substr(0, value_pos));
+			tag_stack.push_front(tag.left(value_pos));
 
 		} else if (tag.begins_with("font=")) {
-			String fnt = tag.substr(5, tag.length()).unquote();
+			String fnt = tag.right(-5).unquote();
 
 			Ref<Font> fc = ResourceLoader::load(fnt, "Font");
 			if (fc.is_valid()) {
@@ -4750,7 +4750,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 			tag_stack.push_front("font");
 
 		} else if (tag.begins_with("font ")) {
-			Vector<String> subtag = tag.substr(2, tag.length()).split(" ");
+			Vector<String> subtag = tag.right(-2).split(" ");
 			_normalize_subtags(subtag);
 
 			Ref<Font> font = theme_cache.normal_font;
@@ -4848,7 +4848,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 			tag_stack.push_front("font");
 
 		} else if (tag.begins_with("outline_size=")) {
-			int fnt_size = tag.substr(13, tag.length()).to_int();
+			int fnt_size = tag.right(-13).to_int();
 			if (fnt_size > 0) {
 				push_outline_size(fnt_size);
 			}
@@ -4987,7 +4987,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 			tag_stack.push_front("pulse");
 			set_process_internal(true);
 		} else if (tag.begins_with("bgcolor=")) {
-			String color_str = tag.substr(8, tag.length()).unquote();
+			String color_str = tag.right(-8).unquote();
 			Color color = Color::from_string(color_str, theme_cache.default_color);
 
 			push_bgcolor(color);
@@ -4995,7 +4995,7 @@ void RichTextLabel::append_text(const String &p_bbcode) {
 			tag_stack.push_front("bgcolor");
 
 		} else if (tag.begins_with("fgcolor=")) {
-			String color_str = tag.substr(8, tag.length()).unquote();
+			String color_str = tag.right(-8).unquote();
 			Color color = Color::from_string(color_str, theme_cache.default_color);
 
 			push_fgcolor(color);
@@ -5424,10 +5424,10 @@ String RichTextLabel::_get_line_text(ItemFrame *p_frame, int p_line, Selection p
 		}
 	}
 	if ((l.from != nullptr) && (p_frame == p_selection.to_frame) && (p_selection.to_item != nullptr) && (p_selection.to_item->index >= l.from->index) && (p_selection.to_item->index < end_idx)) {
-		txt = txt.substr(0, p_selection.to_char);
+		txt = txt.left(p_selection.to_char);
 	}
 	if ((l.from != nullptr) && (p_frame == p_selection.from_frame) && (p_selection.from_item != nullptr) && (p_selection.from_item->index >= l.from->index) && (p_selection.from_item->index < end_idx)) {
-		txt = txt.substr(p_selection.from_char, -1);
+		txt = txt.substr(p_selection.from_char);
 	}
 	return txt;
 }
@@ -6357,7 +6357,7 @@ Dictionary RichTextLabel::parse_expressions_for_values(Vector<String> p_expressi
 				a.append(Color::html(values[j]));
 			} else if (!nodepath.search(values[j]).is_null()) {
 				if (values[j].begins_with("$")) {
-					String v = values[j].substr(1, values[j].length());
+					String v = values[j].right(-1);
 					a.append(NodePath(v));
 				}
 			} else if (!boolean.search(values[j]).is_null()) {
diff --git a/scene/gui/text_edit.cpp b/scene/gui/text_edit.cpp
index 86e726d9dac6..6d6652113423 100644
--- a/scene/gui/text_edit.cpp
+++ b/scene/gui/text_edit.cpp
@@ -1567,7 +1567,7 @@ void TextEdit::_notification(int p_what) {
 				for (int i = 0; i < carets.size(); i++) {
 					String t;
 					if (get_caret_column(i) >= 0) {
-						t = text[get_caret_line(i)].substr(0, get_caret_column(i)) + ime_text + text[get_caret_line(i)].substr(get_caret_column(i), text[get_caret_line(i)].length());
+						t = text[get_caret_line(i)].insert(get_caret_column(i), ime_text);
 					} else {
 						t = ime_text;
 					}
@@ -3565,7 +3565,7 @@ void TextEdit::insert_text_at_caret(const String &p_text, int p_caret) {
 		for (int i = 0; i < carets.size(); i++) {
 			String t;
 			if (get_caret_column(i) >= 0) {
-				t = text[get_caret_line(i)].substr(0, get_caret_column(i)) + ime_text + text[get_caret_line(i)].substr(get_caret_column(i), text[get_caret_line(i)].length());
+				t = text[get_caret_line(i)].insert(get_caret_column(i), ime_text);
 			} else {
 				t = ime_text;
 			}
@@ -7782,9 +7782,9 @@ void TextEdit::_base_insert_text(int p_line, int p_char, const String &p_text, i
 	}
 
 	/* STEP 3: Separate dest string in pre and post text. */
-	String postinsert_text = text[p_line].substr(p_char, text[p_line].size());
+	String postinsert_text = text[p_line].substr(p_char);
 
-	substrings.write[0] = text[p_line].substr(0, p_char) + substrings[0];
+	substrings.write[0] = text[p_line].left(p_char) + substrings[0];
 	substrings.write[substrings.size() - 1] += postinsert_text;
 
 	Vector<Array> bidi_override;
@@ -7850,8 +7850,8 @@ void TextEdit::_base_remove_text(int p_from_line, int p_from_column, int p_to_li
 	ERR_FAIL_COND(p_to_line < p_from_line); // 'from > to'.
 	ERR_FAIL_COND(p_to_line == p_from_line && p_to_column < p_from_column); // 'from > to'.
 
-	String pre_text = text[p_from_line].substr(0, p_from_column);
-	String post_text = text[p_to_line].substr(p_to_column, text[p_to_line].length());
+	String pre_text = text[p_from_line].left(p_from_column);
+	String post_text = text[p_to_line].substr(p_to_column);
 
 	text.remove_range(p_from_line, p_to_line);
 	text.set(p_from_line, pre_text + post_text, structured_text_parser(st_parser, st_args, pre_text + post_text));
diff --git a/scene/main/node.cpp b/scene/main/node.cpp
index e730f47607ec..57a57ca9d4c3 100644
--- a/scene/main/node.cpp
+++ b/scene/main/node.cpp
@@ -1319,7 +1319,7 @@ void Node::_generate_serial_child_name(const Node *p_child, StringName &name) co
 	for (int i = name_string.length() - 1; i >= 0; i--) {
 		char32_t n = name_string[i];
 		if (is_digit(n)) {
-			nums = String::chr(name_string[i]) + nums;
+			nums = String::chr(n) + nums;
 		} else {
 			break;
 		}
@@ -1328,9 +1328,9 @@ void Node::_generate_serial_child_name(const Node *p_child, StringName &name) co
 	String nnsep = _get_name_num_separator();
 	int name_last_index = name_string.length() - nnsep.length() - nums.length();
 
-	// Assign the base name + separator to name if we have numbers preceded by a separator
+	// Assign the base name + separator to name if we have numbers preceded by a separator.
 	if (nums.length() > 0 && name_string.substr(name_last_index, nnsep.length()) == nnsep) {
-		name_string = name_string.substr(0, name_last_index + nnsep.length());
+		name_string = name_string.left(name_last_index + nnsep.length());
 	} else {
 		nums = "";
 	}
diff --git a/scene/resources/packed_scene.cpp b/scene/resources/packed_scene.cpp
index 1f6e453e88e5..5c11f28b328e 100644
--- a/scene/resources/packed_scene.cpp
+++ b/scene/resources/packed_scene.cpp
@@ -699,12 +699,12 @@ Error SceneState::_parse_node(Node *p_owner, Node *p_node, int p_parent_idx, Has
 		} else if (E.type == Variant::ARRAY && E.hint == PROPERTY_HINT_TYPE_STRING) {
 			int hint_subtype_separator = E.hint_string.find(":");
 			if (hint_subtype_separator >= 0) {
-				String subtype_string = E.hint_string.substr(0, hint_subtype_separator);
+				String subtype_string = E.hint_string.left(hint_subtype_separator);
 				int slash_pos = subtype_string.find("/");
 				PropertyHint subtype_hint = PropertyHint::PROPERTY_HINT_NONE;
 				if (slash_pos >= 0) {
 					subtype_hint = PropertyHint(subtype_string.get_slice("/", 1).to_int());
-					subtype_string = subtype_string.substr(0, slash_pos);
+					subtype_string = subtype_string.left(slash_pos);
 				}
 				Variant::Type subtype = Variant::Type(subtype_string.to_int());
 
diff --git a/servers/rendering/shader_preprocessor.cpp b/servers/rendering/shader_preprocessor.cpp
index dbd1374941b2..f3a3bbff1f17 100644
--- a/servers/rendering/shader_preprocessor.cpp
+++ b/servers/rendering/shader_preprocessor.cpp
@@ -674,7 +674,7 @@ void ShaderPreprocessor::process_include(Tokenizer *p_tokenizer) {
 			break;
 		}
 	}
-	path = path.substr(0, path.length() - 1);
+	path = path.left(-1);
 
 	if (path.is_empty() || !p_tokenizer->consume_empty_line()) {
 		set_error(RTR("Invalid path."), line);
@@ -956,7 +956,7 @@ Error ShaderPreprocessor::expand_condition(const String &p_string, int p_line, S
 			String body = state->defines.has(vector_to_string(text)) ? "true" : "false";
 			String temp = result;
 
-			result = result.substr(0, index) + body;
+			result = result.left(index) + body;
 			index_start = result.length();
 			if (index_end > 0) {
 				result += temp.substr(index_end);
@@ -1074,7 +1074,7 @@ bool ShaderPreprocessor::expand_macros_once(const String &p_line, int p_line_num
 				int arg_index_start = 0;
 				int arg_index = 0;
 				while (find_match(body, arg_name, arg_index, arg_index_start)) {
-					body = body.substr(0, arg_index) + args[i] + body.substr(arg_index + arg_name.length(), body.length() - (arg_index + arg_name.length()));
+					body = body.left(arg_index) + args[i] + body.substr(arg_index + arg_name.length());
 					// Manually reset arg_index_start to where the arg value of the define finishes.
 					// This ensures we don't skip the other args of this macro in the string.
 					arg_index_start = arg_index + args[i].length() + 1;
@@ -1083,11 +1083,11 @@ bool ShaderPreprocessor::expand_macros_once(const String &p_line, int p_line_num
 
 			concatenate_macro_body(body);
 
-			result = result.substr(0, index) + " " + body + " " + result.substr(args_end + 1, result.length());
+			result = result.left(index) + " " + body + " " + result.substr(args_end + 1);
 		} else {
 			concatenate_macro_body(body);
 
-			result = result.substr(0, index) + " " + body + " " + result.substr(index + key.length(), result.length() - (index + key.length()));
+			result = result.left(index) + " " + body + " " + result.substr(index + key.length());
 		}
 
 		r_expanded = result;
@@ -1154,7 +1154,7 @@ void ShaderPreprocessor::concatenate_macro_body(String &r_body) {
 			index_start--;
 		}
 
-		r_body = r_body.substr(0, index_start) + r_body.substr(index_end, r_body.length() - index_end);
+		r_body = r_body.left(index_start) + r_body.substr(index_end);
 
 		index_start = r_body.find("##", index_start);
 	}