Sheet names: one quoting rule, Excel 365's, for formulas, ADDRESS, pivots and the Name Box

crates/chart/src/resolve.rs

@@ -41,7 +41,7 @@ fn parse_ref(wb: &Workbook, sheet: usize, formula: &str) -> Option<(usize, Range
     let (sh, addr) = match f.rfind('!') {
         Some(i) => {
             let name = f.get(..i)?.trim();
-            let name = name.strip_prefix('\'').and_then(|n| n.strip_suffix('\'')).map(|n| n.replace("''", "'")).unwrap_or_else(|| name.to_string());
+            let name = gridcraft_core::addr::unquote_sheet(name);
             (wb.sheet_index(&name)?, f.get(i + 1..)?)
         }
         None => (sheet, f),

crates/core/src/addr.rs

@@ -230,6 +230,73 @@ impl fmt::Display for RangeRef {
     }
 }
 
+// ---------------------------------------------------------------- sheet names in references
+
+/// Whether a sheet name needs quotes in a reference (`'My Sheet'!A1`), as Excel 365 writes
+/// formulas. It does when the name:
+/// - starts with a digit or `.` (`1Sheet`, `.x`), or holds anything but letters, digits, `_` and
+///   `.` (`Sheet 1`, `a-b`, `Bob's`);
+/// - reads as an A1 cell (`A1`, `XFD1`, `AB1048576`; not `XFE1` or `AB1048577`, which are off the
+///   sheet);
+/// - reads as an R1C1 reference (`R`, `C`, `RC`, `R1`, `C1`, `R1C`, `RC1`, `R1C1`, in any case);
+/// - is a boolean (`TRUE`, `false`).
+///
+/// Letters of any script stay bare (`Ärger`, `日本`, `Σ`).
+pub fn sheet_needs_quotes(name: &str) -> bool {
+    let Some(first) = name.chars().next() else { return true };
+    if first.is_ascii_digit() || first == '.' || name.chars().any(|c| !(c.is_alphanumeric() || c == '_' || c == '.')) {
+        return true;
+    }
+    CellRef::parse(name).is_some() || is_r1c1_reference(name) || name.eq_ignore_ascii_case("TRUE") || name.eq_ignore_ascii_case("FALSE")
+}
+
+/// `R`, `C`, `RC`, `R1`, `C1`, `R1C`, `RC1`, `R1C1` (any case): what Excel reads as an R1C1 row,
+/// column or cell.
+fn is_r1c1_reference(name: &str) -> bool {
+    let b = name.as_bytes();
+    let mut i = 0;
+    let digits = |i: &mut usize| {
+        while b.get(*i).is_some_and(u8::is_ascii_digit) {
+            *i += 1;
+        }
+    };
+    let r = b.first().is_some_and(|c| c.eq_ignore_ascii_case(&b'R'));
+    if r {
+        i += 1;
+        digits(&mut i);
+    }
+    let c = b.get(i).is_some_and(|c| c.eq_ignore_ascii_case(&b'C'));
+    if c {
+        i += 1;
+        digits(&mut i);
+    }
+    (r || c) && i == b.len()
+}
+
+/// A sheet name as a reference writes it: quoted when [`sheet_needs_quotes`], with each `'`
+/// doubled (`Bob's` → `'Bob''s'`).
+pub fn quote_sheet(name: &str) -> String {
+    if sheet_needs_quotes(name) { format!("'{}'", name.replace('\'', "''")) } else { name.to_string() }
+}
+
+/// The sheet name a reference's sheet part stands for: quotes taken off and `''` read as `'`
+/// (`'Bob''s'` → `Bob's`); a bare name stays as it is.
+pub fn unquote_sheet(text: &str) -> String {
+    match text.strip_prefix('\'').and_then(|t| t.strip_suffix('\'')) {
+        Some(inner) => inner.replace("''", "'"),
+        None => text.to_string(),
+    }
+}
+
+/// `'My Sheet'!A1` → (`Some("My Sheet")`, `A1`); `A1` → (`None`, `A1`). The sheet part is split
+/// at the last `!`, so a quoted name may hold one (`'a!b'!A1`).
+pub fn split_sheet(reference: &str) -> (Option<String>, &str) {
+    match reference.rsplit_once('!') {
+        Some((sheet, body)) => (Some(unquote_sheet(sheet)), body),
+        None => (None, reference),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -292,4 +359,45 @@ mod tests {
         assert_eq!(CellRef::new(0, 0).offset(-1, 0), None);
         assert_eq!(CellRef::new(0, 0).offset_clamped(-5, 3), CellRef::new(0, 3));
     }
+
+    /// Excel 365's quoting of sheet names in formulas (`='name'!A1` typed, `.Formula` read back).
+    #[test]
+    fn sheet_names_are_quoted_like_excel() {
+        for q in [
+            "A1",
+            "XFD1",
+            "AB1048576",
+            "R",
+            "C",
+            "RC",
+            "R1",
+            "C1",
+            "R1C",
+            "RC1",
+            "R1C1",
+            "r2c3",
+            "TRUE",
+            "false",
+            "1Sheet",
+            ".x",
+            "Sheet 1",
+            "a-b",
+            "Bob's",
+            "a#b",
+            "a(b",
+            "",
+        ] {
+            assert!(sheet_needs_quotes(q), "{q} needs quotes");
+        }
+        for b in ["A", "ABC", "XFE1", "AB1048577", "A1A", "x1y", "Sheet1", "Sheet.1", "_x", "x_", "Ärger", "日本", "Σ"] {
+            assert!(!sheet_needs_quotes(b), "{b} stays bare");
+        }
+        assert_eq!(quote_sheet("Bob's"), "'Bob''s'");
+        assert_eq!(quote_sheet("Sheet1"), "Sheet1");
+        assert_eq!(unquote_sheet("'Bob''s'"), "Bob's");
+        assert_eq!(unquote_sheet("Sheet1"), "Sheet1");
+        assert_eq!(split_sheet("'a!b'!A1"), (Some("a!b".to_string()), "A1"));
+        assert_eq!(split_sheet("'Bob''s'!$A$1:$B$2"), (Some("Bob's".to_string()), "$A$1:$B$2"));
+        assert_eq!(split_sheet("A1"), (None, "A1"));
+    }
 }

crates/core/src/lib.rs

@@ -8,6 +8,6 @@ pub mod date;
 pub mod parse;
 pub mod value;
 
-pub use addr::{CellRef, MAX_COLS, MAX_ROWS, RangeRef, col_to_letters, letters_to_col};
+pub use addr::{CellRef, MAX_COLS, MAX_ROWS, RangeRef, col_to_letters, letters_to_col, quote_sheet, sheet_needs_quotes, split_sheet, unquote_sheet};
 pub use date::DateSystem;
 pub use value::{Array, CellError, Value, compare, compare_numbers, compare_text, number_to_text, sort_compare};

crates/engine/src/cmd/edit.rs

@@ -514,7 +514,7 @@ pub(super) fn go_to(s: &mut Session, p: &Json) -> Result<Json> {
         } else if let Some((si, ti)) = d.wb.table(reference) {
             // As in Excel, a table's name stands for its data, without the header and totals rows.
             let t = d.wb.sheet(si).and_then(|sh| sh.tables.get(ti)).map(|t| t.data_range().unwrap_or(t.range).a1()).unwrap_or_default();
-            format!("'{}'!{}", d.wb.sheet(si).map(|s| s.name.clone()).unwrap_or_default(), t)
+            format!("{}!{}", gridcraft_core::quote_sheet(d.wb.sheet(si).map(|s| s.name.as_str()).unwrap_or_default()), t)
         } else {
             return Err(EngineError::Other("Reference isn't valid.".into()));
         }

crates/engine/src/cmd/mod.rs

@@ -269,13 +269,7 @@ pub(crate) fn target_sheet(s: &Session, p: &Json) -> Result<usize> {
     Ok(d.wb.active_sheet)
 }
 
-/// `'My Sheet'!A1` → (`Some("My Sheet")`, `A1`).
-pub(crate) fn split_sheet(r: &str) -> (Option<String>, &str) {
-    match r.rsplit_once('!') {
-        Some((sh, body)) => (Some(sh.trim_matches('\'').replace("''", "'")), body),
-        None => (None, r),
-    }
-}
+pub(crate) use gridcraft_core::addr::split_sheet;
 
 /// A single-cell parameter. Absent or null is `Ok(None)` (callers default it); a value that is not
 /// a cell reference, or that names a sheet other than the active one, is an error rather than a

crates/engine/src/cmd/pivot.rs

@@ -1386,7 +1386,7 @@ fn chart_pivot(s: &Session, p: &Json) -> Result<(usize, usize)> {
         let (sheet, addr) = match t.rsplit_once('!') {
             Some((sh, a)) => {
                 let sh = sh.trim();
-                let sh = sh.strip_prefix('\'').and_then(|x| x.strip_suffix('\'')).map(|x| x.replace("''", "'")).unwrap_or_else(|| sh.to_string());
+                let sh = gridcraft_core::addr::unquote_sheet(sh);
                 (d.wb.sheet_index(&sh), a)
             }
             None => (Some(d.wb.active_sheet), t),

crates/engine/src/cmd/solver.rs

@@ -364,7 +364,7 @@ fn parse_model(s: &Session, p: &Json) -> Result<Model> {
 // ---------------------------------------------------------------- saved model
 
 fn ref_text(sheet_name: &str, cells: &str) -> String {
-    let q = crate::pivot::quote_sheet(sheet_name);
+    let q = gridcraft_core::quote_sheet(sheet_name);
     cells
         .split(',')
         .map(str::trim)
@@ -491,7 +491,7 @@ pub fn saved_model(wb: &Workbook, sheet: usize) -> Option<Json> {
 /// The model as `data.solver` params, in the form [`saved_model`] returns (the objective
 /// qualified by its sheet).
 fn model_params(wb: &Workbook, m: &Model) -> Json {
-    let sheet = wb.sheet(m.sheet).map(|s| crate::pivot::quote_sheet(&s.name)).unwrap_or_default();
+    let sheet = wb.sheet(m.sheet).map(|s| gridcraft_core::quote_sheet(&s.name)).unwrap_or_default();
     let (goal, value) = match m.goal {
         Goal::Max => ("max", 0.0),
         Goal::Min => ("min", 0.0),

crates/engine/src/pivot.rs

@@ -47,25 +47,7 @@ impl Source {
     }
 }
 
-/// `'My Sheet'!A1` → (`Some("My Sheet")`, `A1`).
-fn split_sheet(r: &str) -> (Option<String>, &str) {
-    match r.rsplit_once('!') {
-        Some((sh, body)) => (Some(sh.trim_matches('\'').replace("''", "'")), body),
-        None => (None, r),
-    }
-}
-
-/// Quotes a sheet name for a reference when needed.
-pub fn quote_sheet(name: &str) -> String {
-    if !name.is_empty()
-        && name.chars().all(|c| c.is_alphanumeric() || c == '_' || c == '.')
-        && !name.chars().next().is_some_and(|c| c.is_ascii_digit())
-    {
-        name.to_string()
-    } else {
-        format!("'{}'", name.replace('\'', "''"))
-    }
-}
+pub use gridcraft_core::addr::{quote_sheet, split_sheet};
 
 /// Absolute reference text for a range on a sheet: `Sheet1!$A$1:$E$20`.
 pub fn range_text(sheet_name: &str, r: RangeRef) -> String {
@@ -1511,7 +1493,7 @@ pub fn parse_chart_link(link: &str) -> Option<(String, String)> {
     };
     let (sheet, name) = t.rsplit_once('!')?;
     let sheet = sheet.trim();
-    let sheet = sheet.strip_prefix('\'').and_then(|x| x.strip_suffix('\'')).map(|x| x.replace("''", "'")).unwrap_or_else(|| sheet.to_string());
+    let sheet = gridcraft_core::addr::unquote_sheet(sheet);
     let name = name.trim();
     if sheet.is_empty() || name.is_empty() {
         return None;

crates/engine/src/tests.rs

@@ -814,6 +814,27 @@ fn insert_delete_cells_adjust() {
     assert_eq!(v(&s, "E3"), Value::Number(8.0));
 }
 
+/// One rule quotes sheet names everywhere (Excel 365's): a PivotTable on sheet `A1` names its
+/// source `'A1'!$A$1:$B$3`, and a formula into sheet `.x` keeps its quotes, which Excel needs to
+/// open the file at all.
+#[test]
+fn sheet_names_are_quoted_by_excels_rule_everywhere() {
+    let mut s = s();
+    s.execute("home.insertSheet", json!({"name": "A1"})).unwrap();
+    s.execute("range.setValues", json!({"range": "'A1'!A1", "values": [["K", "V"], ["a", 1], ["b", 2]]})).unwrap();
+    // From the current region, the engine writes the source text itself.
+    s.execute("selection.set", json!({"cell": "A2"})).unwrap();
+    s.execute("insert.pivotTable", json!({"rows": ["K"], "values": ["V"]})).unwrap();
+    let list = s.execute("pivot.list", json!({})).unwrap();
+    assert_eq!(list[0]["source"], "'A1'!$A$1:$B$3", "{list}");
+    s.execute("home.insertSheet", json!({"name": ".x"})).unwrap();
+    s.execute("cell.set", json!({"cell": "A1", "sheet": ".x", "input": "5"})).unwrap();
+    s.execute("home.insertSheet", json!({"name": "R1C1"})).unwrap();
+    s.execute("cell.set", json!({"cell": "'A1'!C1", "input": "='.x'!A1+'R1C1'!A1"})).unwrap();
+    let got = s.execute("cell.get", json!({"cell": "'A1'!C1"})).unwrap();
+    assert_eq!(got["formula"], "='.x'!A1+'R1C1'!A1");
+}
+
 #[test]
 fn sort_and_filter() {
     let mut s = s();

crates/formula/src/printer.rs

@@ -6,22 +6,7 @@ use gridcraft_core::{CellRef, col_to_letters, number_to_text};
 
 use crate::ast::*;
 
-/// Whether a sheet name needs quotes in a reference.
-pub fn needs_quotes(name: &str) -> bool {
-    if name.is_empty() {
-        return true;
-    }
-    let first = name.chars().next().unwrap_or('_');
-    if first.is_ascii_digit() || name.chars().any(|c| !(c.is_alphanumeric() || c == '_' || c == '.')) {
-        return true;
-    }
-    // Looks like a cell ref (`A1`, `R1C1`) or a boolean.
-    gridcraft_core::CellRef::parse(name).is_some() || name.eq_ignore_ascii_case("TRUE") || name.eq_ignore_ascii_case("FALSE")
-}
-
-pub fn quote_sheet(name: &str) -> String {
-    if needs_quotes(name) { format!("'{}'", name.replace('\'', "''")) } else { name.to_string() }
-}
+pub use gridcraft_core::addr::{quote_sheet, sheet_needs_quotes as needs_quotes};
 
 fn sheet_prefix(s: &SheetSel) -> String {
     match s {

crates/functions/src/lookup.rs

@@ -5,7 +5,7 @@
 
 use std::cmp::Ordering;
 
-use gridcraft_core::{Array, CellError, Value, col_to_letters, compare};
+use gridcraft_core::{Array, CellError, Value, col_to_letters, compare, sheet_needs_quotes};
 
 use crate::criteria::{has_wildcards, lookup_equal};
 use crate::util::{R, arg, array_val, as_array, has, int, num, opt_bool, opt_int, opt_num, text, text_val};
@@ -415,10 +415,11 @@ fn transpose(args: &[Arg], _c: &mut dyn Ctx) -> R<Value> {
 
 /// Sheet text for ADDRESS, quoted when it needs to be. A `[Book]Sheet` prefix is judged by its
 /// parts (Microsoft's ADDRESS examples: `"[Book1]Sheet1"` stays bare, `"EXCEL SHEET"` becomes
-/// `'EXCEL SHEET'`).
+/// `'EXCEL SHEET'`). The sheet follows the formula rule ([`sheet_needs_quotes`]: `'A1'!$A$1`,
+/// `'R1C1'!$A$1`), except that Excel 365's ADDRESS leaves `TRUE` and `FALSE` bare.
 fn quote_sheet(s: &str) -> String {
     let word = |p: &str| p.chars().all(|c| c.is_alphanumeric() || c == '_' || c == '.');
-    let bare_sheet = |p: &str| !p.is_empty() && word(p) && !p.chars().next().is_some_and(|c| c.is_ascii_digit());
+    let bare_sheet = |p: &str| !sheet_needs_quotes(p) || p.eq_ignore_ascii_case("TRUE") || p.eq_ignore_ascii_case("FALSE");
     let plain = match s.strip_prefix('[').and_then(|r| r.split_once(']')) {
         Some((book, sheet)) => !book.is_empty() && word(book) && bare_sheet(sheet),
         None => bare_sheet(s),
@@ -791,6 +792,13 @@ mod tests {
         for (r, c, abs, a1, sheet, want) in cases {
             is_text(ev("ADDRESS", vec![n(r), n(c), n(abs), b(a1), t(sheet)]), want);
         }
+        // Excel 365 (ADDRESS(1,1,1,TRUE,name)): names that read as cells or R1C1 references are
+        // quoted, booleans are not.
+        for (sheet, want) in
+            [("A1", "'A1'!$A$1"), ("R1C1", "'R1C1'!$A$1"), ("RC", "'RC'!$A$1"), (".x", "'.x'!$A$1"), ("XFE1", "XFE1!$A$1"), ("TRUE", "TRUE!$A$1")]
+        {
+            is_text(ev("ADDRESS", vec![n(1.0), n(1.0), n(1.0), b(true), t(sheet)]), want);
+        }
         is_text(ev("ADDRESS", vec![n(1048576.0), n(16384.0)]), "$XFD$1048576");
         is_err(ev("ADDRESS", vec![n(0.0), n(1.0)]), CellError::Value);
         is_err(ev("ADDRESS", vec![n(1.0), n(16385.0)]), CellError::Value);

crates/ui-egui/src/lib.rs

@@ -1219,11 +1219,8 @@ impl SheetApp {
         if let Some(sh) = sh {
             for n in &d.wb.names {
                 let target = n.formula.replace('$', "");
-                let (sheet, body) = match target.rsplit_once('!') {
-                    Some((s, b)) => (Some(s.trim_matches('\'').to_string()), b.to_string()),
-                    None => (None, target.clone()),
-                };
-                if sheet.as_deref().is_none_or(|s| s.eq_ignore_ascii_case(&sh.name)) && RangeRef::parse(&body) == Some(cur) {
+                let (sheet, body) = gridcraft_engine::core::addr::split_sheet(&target);
+                if sheet.as_deref().is_none_or(|s| s.eq_ignore_ascii_case(&sh.name)) && RangeRef::parse(body) == Some(cur) {
                     return n.name.clone();
                 }
             }

crates/ui-egui/tests/name_box.rs

@@ -0,0 +1,17 @@
+//! The Name Box shows a defined name when the selection is exactly its range, also on a sheet
+//! whose name holds an apostrophe (`'Bob''s'!$A$1:$B$2`).
+
+use gridcraft_engine::Session;
+use gridcraft_ui_egui::{Services, SheetApp};
+use serde_json::json;
+
+#[test]
+fn name_box_shows_a_name_on_a_sheet_with_an_apostrophe() {
+    let mut session = Session::new();
+    session.new_workbook();
+    session.execute("home.insertSheet", json!({"name": "Bob's"})).unwrap();
+    session.execute("formulas.defineName", json!({"name": "Totals", "refersTo": "='Bob''s'!$A$1:$B$2"})).unwrap();
+    session.execute("selection.set", json!({"range": "A1:B2"})).unwrap();
+    let app = SheetApp::new(session, Services::default());
+    assert_eq!(app.name_box_text(), "Totals");
+}

crates/xlsx/src/array_formula.rs

@@ -53,7 +53,7 @@ impl Scan<'_> {
                     return *shape;
                 }
                 let (scope, bare) = match n.rsplit_once('!') {
-                    Some((s, name)) => (self.wb.sheet_index(&s.trim_matches('\'').replace("''", "'")).unwrap_or(sheet), name),
+                    Some((s, name)) => (self.wb.sheet_index(&gridcraft_core::addr::unquote_sheet(s)).unwrap_or(sheet), name),
                     None => (sheet, n.as_str()),
                 };
                 if let Some(def) = self.wb.name(bare, scope) {

crates/xlsx/src/pivot.rs

@@ -373,10 +373,7 @@ fn resolve_source(wb: &Workbook, text: &str, default_sheet: usize, depth: u32) -
         let xml = if valid { format!("<worksheetSource name=\"{}\"/>", esc_attr(&tb.name)) } else { range_source_xml(&sh.name, r) };
         return Some((si, r, xml));
     }
-    let (sheet, body) = match t.rsplit_once('!') {
-        Some((sh, body)) => (Some(sh.trim_matches('\'').replace("''", "'")), body),
-        None => (None, t),
-    };
+    let (sheet, body) = gridcraft_core::addr::split_sheet(t);
     if let Some(r) = RangeRef::parse(body) {
         let si = match &sheet {
             Some(n) => wb.sheet_index(n)?,

docs/file-format-parity.md

@@ -1,6 +1,6 @@
 # File-format parity
 
-> **Last reviewed:** 2026-10-11 · **Last updated:** 2026-10-12 · **Change:** trivial (XLSX chart sheets are kept on save (#727); previously minor: CSV save keeps the separator it was opened with) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-11 · **Last updated:** 2026-10-13 · **Change:** minor (sheet names in formulas are quoted by one rule, Excel 365's; a formula into a sheet named .x no longer makes the file unopenable in Excel) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
 
 Every format Excel opens or saves, with GridCraft's read and write support, fidelity and tests.
 Part of [target-app-parity.md](target-app-parity.md). **File formats: ~50% ready, 80–120 h**
@@ -85,6 +85,7 @@ refresh. Counted under features (Get & Transform, ~10% ready), not here.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-13 | minor | Sheet names: one quoting rule (`gridcraft_core::addr`: `quote_sheet`, `split_sheet`, `unquote_sheet`), Excel 365's: names that read as A1 cells, R1C1 references (`R`, `RC`, `R1C1`…) or booleans, or start with a digit or `.`, are quoted. A formula into a sheet named `.x` was written `.x!A1`, which Excel 365 couldn't open even with repair; a PivotTable on sheet `A1` named its source `A1!$A$1:$B$3`. |
 | 2026-10-12 | trivial | XLSX chart sheets are kept on save (#727) |
 | 2026-10-12 | trivial | CSV and TSV export is capped at 512 MiB and refuses a larger used range with an error (#528) |
 | 2026-10-12 | minor | `.xls` import reads the calculation settings (CalcMode, CalcIter, CalcCount, CalcDelta, CalcPrecision), so circular models calculate instead of showing `#CIRC!` (#724) |