diff --git a/core/modules/parsers/parseutils.js b/core/modules/parsers/parseutils.js
index 18bcdd5164..fff8589cbd 100644
--- a/core/modules/parsers/parseutils.js
+++ b/core/modules/parsers/parseutils.js
@@ -221,7 +221,7 @@ exports.parseMacroInvocationAsTransclusion = function(source,pos) {
orderedAttributes: []
};
// Define our regexps
- var reVarName = /([^\s>"'=:]+)/y;
+ var reVarName = /([^\s>"'=]+)/y;
// Skip whitespace
pos = $tw.utils.skipWhiteSpace(source,pos);
// Look for a double opening angle bracket
@@ -269,7 +269,7 @@ exports.parseMVVReferenceAsTransclusion = function(source,pos) {
orderedAttributes: []
};
// Define our regexps
- var reVarName = /([^\s>"'=:)]+)/y;
+ var reVarName = /([^\s>"'=)]+)/y;
// Skip whitespace
pos = $tw.utils.skipWhiteSpace(source,pos);
// Look for a double opening parenthesis
diff --git a/editions/test/tiddlers/tests/data/multi-valued-variables/AttributeColonVarName.tid b/editions/test/tiddlers/tests/data/multi-valued-variables/AttributeColonVarName.tid
new file mode 100644
index 0000000000..6b37b5f0b8
--- /dev/null
+++ b/editions/test/tiddlers/tests/data/multi-valued-variables/AttributeColonVarName.tid
@@ -0,0 +1,15 @@
+title: MultiValuedVariables/AttributeColonVarName
+description: ((var)) on a widget attribute may name a variable containing a colon (#10013)
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\whitespace trim
+<$let my:list={{{ [range[2]] }}}>
+<$text text=((my:list))/>
+
++
+title: ExpectedResult
+
+1
\ No newline at end of file
diff --git a/editions/test/tiddlers/tests/data/parse-backcompat/macrocall-colon-varname-and-param.tid b/editions/test/tiddlers/tests/data/parse-backcompat/macrocall-colon-varname-and-param.tid
new file mode 100644
index 0000000000..65f6f7ab8e
--- /dev/null
+++ b/editions/test/tiddlers/tests/data/parse-backcompat/macrocall-colon-varname-and-param.tid
@@ -0,0 +1,16 @@
+title: Parse/BackCompat/MacrocallColonVarNameAndParam
+description: A colon-named procedure or macro accepts named parameters with either separator (#10013)
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\procedure object:method(p) COLONPROC(<>)
+\define object:macro(p) COLONMACRO($p$)
+<>
+<>
+<>
++
+title: ExpectedResult
+
+COLONPROC(V)
COLONPROC(V)
COLONMACRO(V)
\ No newline at end of file
diff --git a/editions/test/tiddlers/tests/data/parse-backcompat/macrocall-colon-varname-as-value.tid b/editions/test/tiddlers/tests/data/parse-backcompat/macrocall-colon-varname-as-value.tid
new file mode 100644
index 0000000000..b3c608bfcd
--- /dev/null
+++ b/editions/test/tiddlers/tests/data/parse-backcompat/macrocall-colon-varname-as-value.tid
@@ -0,0 +1,16 @@
+title: Parse/BackCompat/MacrocallColonVarNameAsValue
+description: A colon-named procedure or macro may be an =-separated parameter value (#10013)
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\procedure object:method() COLONPROC
+\define object:macro() COLONMACRO
+\procedure host(a) [a=<>]
+<>>>
+<>>>
++
+title: ExpectedResult
+
+[a=COLONPROC]
[a=COLONMACRO]
\ No newline at end of file
diff --git a/editions/test/tiddlers/tests/data/parse-backcompat/macrocall-colon-varname.tid b/editions/test/tiddlers/tests/data/parse-backcompat/macrocall-colon-varname.tid
new file mode 100644
index 0000000000..08abc9ecc7
--- /dev/null
+++ b/editions/test/tiddlers/tests/data/parse-backcompat/macrocall-colon-varname.tid
@@ -0,0 +1,13 @@
+title: Parse/BackCompat/MacrocallColonVarName
+description: Macro call variable name may contain a colon (#10013)
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\procedure object:method() VAL
+<>
++
+title: ExpectedResult
+
+VAL
\ No newline at end of file
diff --git a/editions/test/tiddlers/tests/data/parse-backcompat/widget-attr-macro-colon-varname.tid b/editions/test/tiddlers/tests/data/parse-backcompat/widget-attr-macro-colon-varname.tid
new file mode 100644
index 0000000000..40abe57715
--- /dev/null
+++ b/editions/test/tiddlers/tests/data/parse-backcompat/widget-attr-macro-colon-varname.tid
@@ -0,0 +1,13 @@
+title: Parse/BackCompat/WidgetAttrMacroColonVarName
+description: Widget attribute macro value may name a variable containing a colon (#10013)
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\procedure object:method() VAL
+<$text text=<>/>
++
+title: ExpectedResult
+
+VAL
\ No newline at end of file
diff --git a/editions/tw5.com/tiddlers/releasenotes/5.5.0/#10014-colon-in-variable-names.tid b/editions/tw5.com/tiddlers/releasenotes/5.5.0/#10014-colon-in-variable-names.tid
new file mode 100644
index 0000000000..f39e29308d
--- /dev/null
+++ b/editions/tw5.com/tiddlers/releasenotes/5.5.0/#10014-colon-in-variable-names.tid
@@ -0,0 +1,34 @@
+title: $:/changenotes/5.5.0/#10014
+created: 20260906223932000
+modified: 20260906223932000
+description: Variable names may contain a colon in call and multi-valued variable syntax
+release: 5.5.0
+tags: $:/tags/ChangeNote
+change-type: bugfix
+change-category: hackability
+github-links: https://github.com/TiddlyWiki/TiddlyWiki5/pull/10014
+github-contributors: pmario
+
+A variable name may contain a colon in the `<<...>>` call syntax: standalone, with named parameters using either separator, as a nested call, and as a widget attribute value.
+
+```
+\procedure obj:plain() Hello
+\procedure greet:person(name) Hello <>
+\procedure obj:host(a) [<>]
+
+<>
+<>
+<>
+<>>>
+> >
+```
+
+An `((...))` multi-valued variable reference used as a widget attribute value accepts the same names as the inline syntax:
+
+```
+<$let my:list={{{ [range[3]] }}}>
+<$text text=((my:list))/>
+
+```
+
+Fixed issue: [[#10013|https://github.com/TiddlyWiki/TiddlyWiki5/issues/10013]]