feat: add no-space-in-emphasis rule

README.md

@@ -102,6 +102,7 @@ export default defineConfig([
 | [`no-missing-link-fragments`](./docs/rules/no-missing-link-fragments.md) | Disallow link fragments that do not reference valid headings | yes |
 | [`no-multiple-h1`](./docs/rules/no-multiple-h1.md) | Disallow multiple H1 headings in the same document | yes |
 | [`no-reversed-media-syntax`](./docs/rules/no-reversed-media-syntax.md) | Disallow reversed link and image syntax | yes |
+| [`no-space-in-emphasis`](./docs/rules/no-space-in-emphasis.md) | Disallow spaces around emphasis markers | yes |
 | [`require-alt-text`](./docs/rules/require-alt-text.md) | Require alternative text for images | yes |
 | [`table-column-count`](./docs/rules/table-column-count.md) | Disallow data rows in a GitHub Flavored Markdown table from having more cells than the header row | yes |
 <!-- Rule Table End -->

docs/rules/no-space-in-emphasis.md

@@ -0,0 +1,43 @@
+# no-space-in-emphasis
+
+Disallow spaces around emphasis markers.
+
+## Background
+
+In Markdown, emphasis (bold and italic) is created using asterisks (`*`) or underscores (`_`), and strikethrough is created using tildes (`~`). The emphasis markers must be directly adjacent to the text they're emphasizing, with no spaces between the markers and the text. When spaces are present, the emphasis is not rendered correctly.
+
+## Rule Details
+
+This rule warns when it finds emphasis markers that have spaces between the markers and the text they're emphasizing.
+
+Examples of **incorrect** code for this rule:
+
+```markdown
+<!-- eslint markdown/no-space-in-emphasis: "error" -->
+
+Here is some ** bold ** text.
+Here is some * italic * text.
+Here is some __ bold __ text.
+Here is some _ italic _ text.
+Here is some ~~ strikethrough ~~ text.
+```
+
+Examples of **correct** code for this rule:
+
+```markdown
+<!-- eslint markdown/no-space-in-emphasis: "error" -->
+
+Here is some **bold** text.
+Here is some *italic* text.
+Here is some __bold__ text.
+Here is some _italic_ text.
+Here is some ~~strikethrough~~ text.
+```
+
+## When Not to Use It
+
+If you aren't concerned with proper emphasis rendering in your Markdown documents, you can safely disable this rule.
+
+## Prior Art
+
+* [MD037 - Spaces inside emphasis markers](https://github.com/DavidAnson/markdownlint/blob/main/doc/md037.md)

src/rules/no-space-in-emphasis.js

@@ -0,0 +1,145 @@
+/**
+ * @fileoverview Rule to prevent spaces around emphasis markers in Markdown.
+ * @author Pixel998
+ */
+
+//-----------------------------------------------------------------------------
+// Imports
+//-----------------------------------------------------------------------------
+
+import { findOffsets } from "../util.js";
+
+//-----------------------------------------------------------------------------
+// Type Definitions
+//-----------------------------------------------------------------------------
+
+/**
+ * @typedef {import("../types.ts").MarkdownRuleDefinition<{ RuleOptions: []; }>}
+ * NoSpaceInEmphasisRuleDefinition
+ */
+
+//-----------------------------------------------------------------------------
+// Helpers
+//-----------------------------------------------------------------------------
+
+const markerPattern = /(?<!\\)(\*\*\*|\*\*|\*|___|__|_|~~|~)/gu;
+const whitespacePattern = /\s/u;
+
+/**
+ * Finds all emphasis markers in the text
+ * @param {string} text The text to search
+ * @returns {Array<{marker: string, start: number, end: number}>} Array of emphasis markers
+ */
+function findEmphasisMarkers(text) {
+	const markers = [];
+	let match;
+
+	while ((match = markerPattern.exec(text)) !== null) {
+		const marker = match[1];
+		const start = match.index;
+		const end = start + marker.length;
+
+		markers.push({ marker, start, end });
+	}
+
+	return markers;
+}
+
+//-----------------------------------------------------------------------------
+// Rule Definition
+//-----------------------------------------------------------------------------
+
+/** @type {NoSpaceInEmphasisRuleDefinition} */
+export default {
+	meta: {
+		type: "problem",
+
+		docs: {
+			recommended: true,
+			description: "Disallow spaces around emphasis markers",
+			url: "https://github.com/eslint/markdown/blob/main/docs/rules/no-space-in-emphasis.md",
+		},
+
+		fixable: "whitespace",
+
+		messages: {
+			spaceInEmphasis: "Unexpected space around emphasis marker.",
+		},
+	},
+
+	create(context) {
+		const { sourceCode } = context;
+
+		return {
+			text(node) {
+				const text = sourceCode.getText(node);
+				const markers = findEmphasisMarkers(text);
+
+				for (let i = 0; i < markers.length - 1; i += 2) {
+					const startMarker = markers[i];
+					const endMarker = markers[i + 1];
+
+					if (startMarker.marker !== endMarker.marker) {
+						continue;
+					}
+
+					const hasStartSpace = whitespacePattern.test(
+						text[startMarker.end],
+					);
+					const hasEndSpace = whitespacePattern.test(
+						text[endMarker.start - 1],
+					);
+
+					if (hasStartSpace || hasEndSpace) {
+						const {
+							lineOffset: startLineOffset,
+							columnOffset: startColumnOffset,
+						} = findOffsets(text, startMarker.start);
+						const {
+							lineOffset: endLineOffset,
+							columnOffset: endColumnOffset,
+						} = findOffsets(text, endMarker.end);
+
+						const startLine =
+							node.position.start.line + startLineOffset;
+						const endLine =
+							node.position.start.line + endLineOffset;
+						const startColumn =
+							node.position.start.column + startColumnOffset;
+						const endColumn =
+							node.position.start.column + endColumnOffset;
+
+						context.report({
+							loc: {
+								start: { line: startLine, column: startColumn },
+								end: { line: endLine, column: endColumn },
+							},
+							messageId: "spaceInEmphasis",
+							fix(fixer) {
+								const betweenText = text.slice(
+									startMarker.end,
+									endMarker.start,
+								);
+
+								const fixedText =
+									startMarker.marker +
+									betweenText.trim() +
+									endMarker.marker;
+
+								const nodeStart = node.position.start.offset;
+								const relativeStart =
+									nodeStart + startMarker.start;
+								const relativeEnd = nodeStart + endMarker.end;
+
+								return fixer.replaceTextRange(
+									[relativeStart, relativeEnd],
+									fixedText,
+								);
+							},
+						});
+					}
+				}
+			},
+		};
+	},
+};