diff --git a/packages/mermaid/package.json b/packages/mermaid/package.json
index 1d9e12c83..fd10eb0d6 100644
--- a/packages/mermaid/package.json
+++ b/packages/mermaid/package.json
@@ -52,7 +52,7 @@
     "postinstall": "pnpm antlr:generate",
     "checkCircle": "npx madge --circular ./src",
     "antlr:sequence:clean": "rimraf src/diagrams/sequence/parser/antlr/generated",
-    "antlr:sequence": "pnpm run antlr:sequence:clean && antlr4ng -Dlanguage=TypeScript -Xexact-output-dir -o src/diagrams/sequence/parser/antlr/generated src/diagrams/sequence/parser/antlr/SequenceLexer.g4 src/diagrams/sequence/parser/antlr/SequenceParser.g4",
+    "antlr:sequence": "pnpm run antlr:sequence:clean && antlr4ng -Dlanguage=TypeScript -Xexact-output-dir -lib src/diagrams/common/parser/antlr -o src/diagrams/sequence/parser/antlr/generated src/diagrams/sequence/parser/antlr/SequenceLexer.g4 src/diagrams/sequence/parser/antlr/SequenceParser.g4",
     "antlr:class:clean": "rimraf src/diagrams/class/parser/antlr/generated",
     "antlr:class": "pnpm run antlr:class:clean && antlr4ng -Dlanguage=TypeScript -Xexact-output-dir -o src/diagrams/class/parser/antlr/generated src/diagrams/class/parser/antlr/ClassLexer.g4 src/diagrams/class/parser/antlr/ClassParser.g4",
     "antlr:flowchart:clean": "rimraf src/diagrams/flowchart/parser/antlr/generated",
diff --git a/packages/mermaid/src/diagram-api/regexes.ts b/packages/mermaid/src/diagram-api/regexes.ts
index 66873be31..e63b9ee2e 100644
--- a/packages/mermaid/src/diagram-api/regexes.ts
+++ b/packages/mermaid/src/diagram-api/regexes.ts
@@ -3,7 +3,8 @@
 // Note that JS doesn't support the "\A" anchor, which means we can't use
 // multiline mode.
 // Relevant YAML spec: https://yaml.org/spec/1.2.2/#914-explicit-documents
-export const frontMatterRegex = /^\uFEFF?[\t ]*-{3}[\t ]*\r?\n([\S\s]*?)\r?\n-{3}[\t ]*(?:\r?\n|$)/;
+export const frontMatterRegex =
+  /^\uFEFF?[\t ]*-{3}[\t ]*\r?\n([\S\s]*?)\r?\n {0,2}-{3}[\t ]*(?:\r?\n|$)/;
 
 export const directiveRegex =
   /%{2}{\s*(?:(\w+)\s*:|(\w+))\s*(?:(\w+)|((?:(?!}%{2}).|\r?\n)*))?\s*(?:}%{2})?/gi;
diff --git a/packages/mermaid/src/diagrams/common/parser/antlr/HeaderCommon.g4 b/packages/mermaid/src/diagrams/common/parser/antlr/HeaderCommon.g4
new file mode 100644
index 000000000..91d20bc59
--- /dev/null
+++ b/packages/mermaid/src/diagrams/common/parser/antlr/HeaderCommon.g4
@@ -0,0 +1,25 @@
+lexer grammar HeaderCommon;
+
+@members {
+  // headerMode is true until the diagram header keyword is seen
+  protected headerMode = true;
+  // Helper to disable header mode from delegator lexers on diagram start
+  protected disableHeaderMode(): void { this.headerMode = false; }
+}
+
+// Header directives: only before the diagram header keyword has been seen
+// Accept optional leading spaces/tabs on the line before the directive
+HEADER_DIRECTIVE: { this.headerMode }? [ \t]* '%%{' .*? '}%%';
+
+// YAML front matter (allowed only before the diagram header)
+// Use a dedicated mode to consume until the closing '---' line
+FRONTMATTER: { this.headerMode }? [ \t]* '---' [ \t]* ('\r'? '\n') -> pushMode(YAML_MODE);
+
+mode YAML_MODE;
+YAML_END: [ \t]* '---' [ \t]* ('\r'? '\n') -> popMode, skip;
+YAML_CONTENT: . -> skip;
+
+// Comments (skip) - simple, broad handling; rely on longest-match to keep HEADER_DIRECTIVE intact
+HASH_COMMENT: '#' ~[\r\n]* -> skip;
+PERCENT_COMMENT: '%%' ~[\r\n]* -> skip;
+
diff --git a/packages/mermaid/src/diagrams/sequence/parser/antlr/SequenceLexer.g4 b/packages/mermaid/src/diagrams/sequence/parser/antlr/SequenceLexer.g4
index a29f11507..e76fbf8ef 100644
--- a/packages/mermaid/src/diagrams/sequence/parser/antlr/SequenceLexer.g4
+++ b/packages/mermaid/src/diagrams/sequence/parser/antlr/SequenceLexer.g4
@@ -1,27 +1,17 @@
 lexer grammar SequenceLexer;
+import HeaderCommon;
 tokens { AS }
 
-@members {
-  // headerMode is true until the diagram header (sequenceDiagram) is seen
-  private headerMode = true;
-}
 
-// Header directives: handle %%{ ... }%% only before the diagram header
-// Accept optional leading spaces/tabs on the line before the directive
-HEADER_DIRECTIVE: { this.headerMode }? [ \t]* '%%{' .*? '}%%';
-
-// Comments (skip) - avoid consuming '%%{' which starts a directive
-HASH_COMMENT: '#' ~[\r\n]* -> skip;
-PERCENT_COMMENT1: '%%' ~['{'] ~[\r\n]* -> skip;
-PERCENT_COMMENT2: ~[}] '%%' ~[\r\n]* -> skip;
 
 // Whitespace and newline
 
-// YAML front matter (allowed before the diagram header)
-FRONTMATTER: { this.headerMode }? [ \t]* '---' [ \t]* ('\r'? '\n') .*? ('\r'? '\n') [ \t]* '---' [ \t]* ('\r'? '\n');
 
 NEWLINE: ('\r'? '\n')+;
 WS: [ \t]+ -> skip;
+// Top-level comments (also defined in HeaderCommon, duplicated here to ensure availability post-header)
+HASH_COMMENT_TOP: '#' ~[\r\n]* -> skip;
+PERCENT_COMMENT_TOP: '%%' ~[\r\n]* -> skip;
 
 // Punctuation and simple symbols
 COMMA: ',';
diff --git a/scripts/antlr-generate.mts b/scripts/antlr-generate.mts
index 298394959..cfdfdd4ba 100644
--- a/scripts/antlr-generate.mts
+++ b/scripts/antlr-generate.mts
@@ -141,12 +141,19 @@ function generateAntlrFiles(grammar: GrammarInfo): void {
     cleanGeneratedDir(outputDir);
     ensureGeneratedDir(outputDir);
 
+    // Determine common header lib path for imported grammars
+    const cwd = process.cwd();
+    const commonLibPath = cwd.endsWith('/packages/mermaid')
+      ? 'src/diagrams/common/parser/antlr'
+      : 'packages/mermaid/src/diagrams/common/parser/antlr';
+
     // Generate ANTLR files
     const command = [
       'antlr-ng',
       '-Dlanguage=TypeScript',
       '-l',
       '-v',
+      `--lib "${commonLibPath}"`,
       `-o "${outputDir}"`,
       `"${lexerFile}"`,
       `"${parserFile}"`,