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+---
+epic: 7
+story: 7.3
+title: "Syntax Highlighting"
+status: draft
+---
+
+## Epic 7 — macOS App (Swift/SwiftUI)
+**Goal:** A beautiful, native macOS app that feels like it belongs on the platform.
+
+### Story 7.3: Syntax Highlighting
+
+As a **macOS user**,
+I want syntax highlighting in the editor that colors numbers, operators, units, variables, comments, and errors,
+So that I can visually parse my calculations quickly and spot mistakes at a glance.
+
+**Acceptance Criteria:**
+
+**Given** the user types `monthly_rent = $1250 // housing cost`
+**When** the text is rendered in the editor
+**Then** `monthly_rent` is colored as a variable, `=` as an operator, `$1250` as a number/currency, and `// housing cost` as a comment
+**And** each category has a distinct color
+
+**Given** the user types an invalid expression that produces an error
+**When** the text is rendered
+**Then** the erroneous portion is highlighted with an error color (e.g., red underline or red text)
+
+**Given** the system appearance is set to Light mode
+**When** the editor renders syntax highlighting
+**Then** colors are appropriate for a light background (sufficient contrast, readable)
+
+**Given** the system appearance is set to Dark mode
+**When** the editor renders syntax highlighting
+**Then** colors are appropriate for a dark background (sufficient contrast, readable)
+
+**Given** the user types or edits text
+**When** characters are inserted or deleted
+**Then** syntax highlighting updates incrementally without visible flicker or lag
+
+**Given** the editor uses a custom `NSTextStorage` subclass
+**When** the engine tokenizer produces token spans for a line
+**Then** the `NSTextStorage` applies the corresponding `NSAttributedString` attributes to match those spans