diff --git a/arduino-ide-extension/src/common/protocol/sketches-service.ts b/arduino-ide-extension/src/common/protocol/sketches-service.ts
index df25a34f3..7953be720 100644
--- a/arduino-ide-extension/src/common/protocol/sketches-service.ts
+++ b/arduino-ide-extension/src/common/protocol/sketches-service.ts
@@ -157,8 +157,6 @@ export namespace Sketch {
   // (non-API) exported for the tests
   export const defaultSketchFolderName = 'sketch';
   // (non-API) exported for the tests
-  export const defaultFallbackFirstChar = '0';
-  // (non-API) exported for the tests
   export const defaultFallbackChar = '_';
   // (non-API) exported for the tests
   export function reservedFilename(name: string): string {
@@ -176,11 +174,11 @@ export namespace Sketch {
   // (non-API) exported for the tests
   export const invalidSketchFolderNameMessage = nls.localize(
     'arduino/sketch/invalidSketchName',
-    'The name must start with a letter or number, followed by letters, numbers, dashes, dots and underscores. Maximum length is 63 characters.'
+    'The name must start with a letter, number, or underscore, followed by letters, numbers, dashes, dots and underscores. Maximum length is 63 characters.'
   );
   const invalidCloudSketchFolderNameMessage = nls.localize(
     'arduino/sketch/invalidCloudSketchName',
-    'The name must start with a letter or number, followed by letters, numbers, dashes, dots and underscores. Maximum length is 36 characters.'
+    'The name must start with a letter, number, or underscore, followed by letters, numbers, dashes, dots and underscores. Maximum length is 36 characters.'
   );
   /**
    * `undefined` if the candidate sketch folder name is valid. Otherwise, the validation error message.
@@ -193,7 +191,7 @@ export namespace Sketch {
     if (validFilenameError) {
       return validFilenameError;
     }
-    return /^[0-9a-zA-Z]{1}[0-9a-zA-Z_\.-]{0,62}$/.test(candidate)
+    return /^[0-9a-zA-Z_]{1}[0-9a-zA-Z_\.-]{0,62}$/.test(candidate)
       ? undefined
       : invalidSketchFolderNameMessage;
   }
@@ -208,7 +206,7 @@ export namespace Sketch {
     if (validFilenameError) {
       return validFilenameError;
     }
-    return /^[0-9a-zA-Z]{1}[0-9a-zA-Z_\.-]{0,35}$/.test(candidate)
+    return /^[0-9a-zA-Z_]{1}[0-9a-zA-Z_\.-]{0,35}$/.test(candidate)
       ? undefined
       : invalidCloudSketchFolderNameMessage;
   }
@@ -252,10 +250,7 @@ export namespace Sketch {
       return defaultSketchFolderName;
     }
     const validName = candidate
-      ? candidate
-          .replace(/^[^0-9a-zA-Z]{1}/g, defaultFallbackFirstChar)
-          .replace(/[^0-9a-zA-Z_]/g, defaultFallbackChar)
-          .slice(0, 63)
+      ? candidate.replace(/[^0-9a-zA-Z_]/g, defaultFallbackChar).slice(0, 63)
       : defaultSketchFolderName;
     if (appendTimestampSuffix) {
       return `${validName.slice(0, 63 - timestampSuffixLength)}${
@@ -283,10 +278,7 @@ export namespace Sketch {
       return defaultSketchFolderName;
     }
     return candidate
-      ? candidate
-          .replace(/^[^0-9a-zA-Z]{1}/g, defaultFallbackFirstChar)
-          .replace(/[^0-9a-zA-Z_]/g, defaultFallbackChar)
-          .slice(0, 36)
+      ? candidate.replace(/[^0-9a-zA-Z_]/g, defaultFallbackChar).slice(0, 36)
       : defaultSketchFolderName;
   }
 
diff --git a/arduino-ide-extension/src/test/browser/workspace-commands.test.ts b/arduino-ide-extension/src/test/browser/workspace-commands.test.ts
index 234515e76..0676ed77a 100644
--- a/arduino-ide-extension/src/test/browser/workspace-commands.test.ts
+++ b/arduino-ide-extension/src/test/browser/workspace-commands.test.ts
@@ -171,22 +171,22 @@ describe('workspace-commands', () => {
     });
 
     it('code files cannot start with number (no extension)', async () => {
-      const actual = await testMe('_invalid');
+      const actual = await testMe('-invalid');
       expect(actual).to.be.equal(Sketch.invalidSketchFolderNameMessage);
     });
 
     it('code files cannot start with number (trailing dot)', async () => {
-      const actual = await testMe('_invalid.');
+      const actual = await testMe('-invalid.');
       expect(actual).to.be.equal(Sketch.invalidSketchFolderNameMessage);
     });
 
     it('code files cannot start with number (trailing dot)', async () => {
-      const actual = await testMe('_invalid.cpp');
+      const actual = await testMe('-invalid.cpp');
       expect(actual).to.be.equal(Sketch.invalidSketchFolderNameMessage);
     });
 
     it('should warn about invalid extension first', async () => {
-      const actual = await testMe('_invalid.xxx');
+      const actual = await testMe('-invalid.xxx');
       expect(actual).to.be.equal(invalidExtensionMessage('.xxx'));
     });
 
@@ -196,7 +196,7 @@ describe('workspace-commands', () => {
     });
 
     it('should ignore non-code filename validation from the spec', async () => {
-      const actual = await testMe('_invalid.json');
+      const actual = await testMe('-invalid.json');
       expect(actual).to.be.empty;
     });
 
diff --git a/arduino-ide-extension/src/test/common/sketches-service.test.ts b/arduino-ide-extension/src/test/common/sketches-service.test.ts
index 917155e58..ae9f2481c 100644
--- a/arduino-ide-extension/src/test/common/sketches-service.test.ts
+++ b/arduino-ide-extension/src/test/common/sketches-service.test.ts
@@ -43,7 +43,7 @@ describe('sketch', () => {
         ['trailing.dots...', false],
         ['no.trailing.dots.._', true],
         ['No Spaces', false],
-        ['_invalidToStartWithUnderscore', false],
+        ['_validToStartWithUnderscore', true],
         ['Invalid+Char.ino', false],
         ['', false],
         ['/', false],
@@ -81,7 +81,7 @@ describe('sketch', () => {
         ['can.contain.dots', true],
         ['-cannot-start-with-dash', false],
         ['.cannot.start.with.dash', false],
-        ['_cannot_start_with_underscore', false],
+        ['_can_start_with_underscore', true],
         ['No Spaces', false],
         ['Invalid+Char.ino', false],
         ['', false],
@@ -108,14 +108,14 @@ describe('sketch', () => {
   describe('toValidSketchFolderName', () => {
     [
       ['', Sketch.defaultSketchFolderName],
-      [' ', Sketch.defaultFallbackFirstChar],
-      ['  ', Sketch.defaultFallbackFirstChar + Sketch.defaultFallbackChar],
+      [' ', Sketch.defaultFallbackChar],
+      ['  ', Sketch.defaultFallbackChar + Sketch.defaultFallbackChar],
       [
         '0123456789012345678901234567890123456789012345678901234567890123',
         '012345678901234567890123456789012345678901234567890123456789012',
       ],
       ['foo bar', 'foo_bar'],
-      ['_foobar', '0foobar'],
+      ['-foobar', '_foobar'],
       ['vAlid', 'vAlid'],
       ['COM1', Sketch.defaultSketchFolderName],
       ['COM1.', 'COM1_'],
@@ -130,20 +130,18 @@ describe('sketch', () => {
     const epochSuffix = Sketch.timestampSuffix(epoch);
     [
       ['', Sketch.defaultSketchFolderName + epochSuffix],
-      [' ', Sketch.defaultFallbackFirstChar + epochSuffix],
+      [' ', Sketch.defaultFallbackChar + epochSuffix],
       [
         '  ',
-        Sketch.defaultFallbackFirstChar +
-          Sketch.defaultFallbackChar +
-          epochSuffix,
+        Sketch.defaultFallbackChar + Sketch.defaultFallbackChar + epochSuffix,
       ],
       [
         '0123456789012345678901234567890123456789012345678901234567890123',
         '0123456789012345678901234567890123456789012' + epochSuffix,
       ],
       ['foo bar', 'foo_bar' + epochSuffix],
-      ['.foobar', '0foobar' + epochSuffix],
-      ['-fooBar', '0fooBar' + epochSuffix],
+      ['.foobar', '_foobar' + epochSuffix],
+      ['-fooBar', '_fooBar' + epochSuffix],
       ['foobar.', 'foobar_' + epochSuffix],
       ['fooBar-', 'fooBar_' + epochSuffix],
       ['fooBar+', 'fooBar_' + epochSuffix],
@@ -164,19 +162,17 @@ describe('sketch', () => {
       ['only_underscore-is+ok.ino', 'only_underscore_is_ok_ino'],
       ['regex++', 'regex__'],
       ['dots...', 'dots___'],
-      ['.dots...', '0dots___'],
-      ['-dashes---', '0dashes___'],
-      ['_underscore___', '0underscore___'],
+      ['.dots...', '_dots___'],
+      ['-dashes---', '_dashes___'],
       ['No Spaces', 'No_Spaces'],
-      ['_startsWithUnderscore', '0startsWithUnderscore'],
       ['Invalid+Char.ino', 'Invalid_Char_ino'],
       ['', 'sketch'],
-      ['/', '0'],
+      ['/', '_'],
       [
         '/-1////////////////////+//////////////-/',
-        '0_1_________________________________',
+        '__1_________________________________',
       ],
-      ['//trash/', '0_trash_'],
+      ['//trash/', '__trash_'],
       [
         '63Length_012345678901234567890123456789012345678901234567890123',
         '63Length_012345678901234567890123456',
diff --git a/i18n/en.json b/i18n/en.json
index e02f7eeb4..b24a89cd4 100644
--- a/i18n/en.json
+++ b/i18n/en.json
@@ -419,11 +419,11 @@
       "editInvalidSketchFolderLocationQuestion": "Do you want to try saving the sketch to a different location?",
       "editInvalidSketchFolderQuestion": "Do you want to try saving the sketch with a different name?",
       "exportBinary": "Export Compiled Binary",
-      "invalidCloudSketchName": "The name must start with a letter or number, followed by letters, numbers, dashes, dots and underscores. Maximum length is 36 characters.",
+      "invalidCloudSketchName": "The name must start with a letter, number, or underscore, followed by letters, numbers, dashes, dots and underscores. Maximum length is 36 characters.",
       "invalidSketchFolderLocationDetails": "You cannot save a sketch into a folder inside itself.",
       "invalidSketchFolderLocationMessage": "Invalid sketch folder location: '{0}'",
       "invalidSketchFolderNameMessage": "Invalid sketch folder name: '{0}'",
-      "invalidSketchName": "The name must start with a letter or number, followed by letters, numbers, dashes, dots and underscores. Maximum length is 63 characters.",
+      "invalidSketchName": "The name must start with a letter, number, or underscore, followed by letters, numbers, dashes, dots and underscores. Maximum length is 63 characters.",
       "moving": "Moving",
       "movingMsg": "The file \"{0}\" needs to be inside a sketch folder named \"{1}\".\nCreate this folder, move the file, and continue?",
       "new": "New Sketch",