diff --git a/src/mentionui.js b/src/mentionui.js
index a1b33a6..e02e937 100644
--- a/src/mentionui.js
+++ b/src/mentionui.js
@@ -640,7 +640,7 @@ export function createRegExp( marker, minimumCharacters ) {
 	const numberOfCharacters = minimumCharacters == 0 ? '*' : `{${ minimumCharacters },}`;
 
 	const openAfterCharacters = env.features.isRegExpUnicodePropertySupported ? '\\p{Ps}\\p{Pi}"\'' : '\\(\\[{"\'';
-	const mentionCharacters = env.features.isRegExpUnicodePropertySupported ? '\\p{L}\\p{N}' : 'a-zA-ZÀ-ž0-9';
+	const mentionCharacters = '\\S';
 
 	// The pattern consists of 3 groups:
 	// - 0 (non-capturing): Opening sequence - start of the line, space or an opening punctuation character like "(" or "\"",
@@ -649,7 +649,7 @@ export function createRegExp( marker, minimumCharacters ) {
 	//
 	// The pattern matches up to the caret (end of string switch - $).
 	//               (0:      opening sequence       )(1:  marker   )(2:                typed mention                 )$
-	const pattern = `(?:^|[ ${ openAfterCharacters }])([${ marker }])([_${ mentionCharacters }]${ numberOfCharacters })$`;
+	const pattern = `(?:^|[ ${ openAfterCharacters }])([${ marker }])([${ mentionCharacters }]${ numberOfCharacters })$`;
 
 	return new RegExp( pattern, 'u' );
 }
diff --git a/tests/manual/mention.js b/tests/manual/mention.js
index a9824ce..9c5a907 100644
--- a/tests/manual/mention.js
+++ b/tests/manual/mention.js
@@ -165,6 +165,12 @@ ClassicEditor
 						'#a01', '#a02', '#a03', '#a04', '#a05', '#a06', '#a07', '#a08', '#a09', '#a10',
 						'#a11', '#a12', '#a13', '#a14', '#a15', '#a16', '#a17', '#a18', '#a19', '#a20'
 					]
+				},
+				{
+					marker: ':',
+					feed: [
+						':+1:', ':-1:', ':@(at-sign):', ':$(dollar-sign):', ':#(hash-sign):'
+					]
 				}
 			]
 		}
diff --git a/tests/manual/mention.md b/tests/manual/mention.md
index 6493d6e..bed4ffc 100644
--- a/tests/manual/mention.md
+++ b/tests/manual/mention.md
@@ -21,6 +21,14 @@ The feeds:
     - ...
     - a20
 
+3. Static list of "special characters" items (`:` marker)
+
+    - :+1:
+    - :-1:
+    - :@(at-sign):
+    - :$(dollar-sign):
+    - :#(hash-sign):
+
 ### Interaction
 
 You can interact with mention panel with keyboard:
diff --git a/tests/mentionui.js b/tests/mentionui.js
index 7a20224..6561ae8 100644
--- a/tests/mentionui.js
+++ b/tests/mentionui.js
@@ -518,14 +518,14 @@ describe( 'MentionUI', () => {
 				env.features.isRegExpUnicodePropertySupported = false;
 				createRegExp( '@', 2 );
 				sinon.assert.calledOnce( regExpStub );
-				sinon.assert.calledWithExactly( regExpStub, '(?:^|[ \\(\\[{"\'])([@])([_a-zA-ZÀ-ž0-9]{2,})$', 'u' );
+				sinon.assert.calledWithExactly( regExpStub, '(?:^|[ \\(\\[{"\'])([@])([\\S]{2,})$', 'u' );
 			} );
 
 			it( 'returns a ES2018 RegExp for browsers supporting Unicode punctuation groups', () => {
 				env.features.isRegExpUnicodePropertySupported = true;
 				createRegExp( '@', 2 );
 				sinon.assert.calledOnce( regExpStub );
-				sinon.assert.calledWithExactly( regExpStub, '(?:^|[ \\p{Ps}\\p{Pi}"\'])([@])([_\\p{L}\\p{N}]{2,})$', 'u' );
+				sinon.assert.calledWithExactly( regExpStub, '(?:^|[ \\p{Ps}\\p{Pi}"\'])([@])([\\S]{2,})$', 'u' );
 			} );
 		} );
 
@@ -1916,7 +1916,11 @@ describe( 'MentionUI', () => {
 						},
 						{
 							marker: '$',
-							feed: [ '$a1', '$a2', '$a3', '$a4', '$a5' ]
+							feed: [
+								'$a1', '$a2', '$a3', '$a4',
+								// A case of mention with a marker character from other feed. See #6398.
+								'$a@'
+							]
 						}
 					]
 				} );
@@ -1971,6 +1975,43 @@ describe( 'MentionUI', () => {
 						expect( mentionsView.items ).to.have.length( 3 );
 					} );
 			} );
+
+			it( 'should show panel for matched marker if it contains the other configured marker', () => {
+				setData( model, '<paragraph>foo []</paragraph>' );
+
+				model.change( writer => {
+					writer.insertText( '@', doc.selection.getFirstPosition() );
+				} );
+
+				return waitForDebounce()
+					.then( () => {
+						expect( panelView.isVisible ).to.be.true;
+						expect( editor.model.markers.has( 'mention' ) ).to.be.true;
+						expect( mentionsView.items ).to.have.length( 3 );
+
+						mentionsView.items.get( 0 ).children.get( 0 ).fire( 'execute' );
+					} )
+					.then( waitForDebounce )
+					.then( () => {
+						expect( panelView.isVisible ).to.be.false;
+						expect( editor.model.markers.has( 'mention' ) ).to.be.false;
+
+						model.change( writer => {
+							writer.insertText( '$a', doc.selection.getFirstPosition() );
+						} );
+					} )
+					.then( waitForDebounce )
+					.then( () => {
+						model.change( writer => {
+							writer.insertText( '@', doc.selection.getFirstPosition() );
+						} );
+					} )
+					.then( waitForDebounce )
+					.then( () => {
+						expect( panelView.isVisible ).to.be.true;
+						expect( editor.model.markers.has( 'mention' ) ).to.be.true;
+					} );
+			} );
 		} );
 
 		function testExecuteKey( name, keyCode, feedItems ) {