fix(core): detect showLineNumbers anywhere in the meta string

.changeset/fair-ads-begin.md

@@ -0,0 +1,5 @@
+---
+"rehype-pretty-code": patch
+---
+
+fix: detect `showLineNumbers` wherever it appears in the meta string, not only before the highlight annotations, while ignoring the word when it is part of a character highlight such as `/foo showLineNumbers bar/` (#204)

packages/core/src/index.ts

@@ -11,7 +11,6 @@ import rangeParser from 'parse-numeric-range';
 import { unified, type Transformer } from 'unified';
 import rehypeParse from 'rehype-parse';
 import { charsHighlighter } from './chars/charsHighlighter';
-import { reverseString } from './chars/utils';
 import {
   isElement,
   isText,
@@ -23,6 +22,7 @@ import {
   getThemeNames,
   replaceLineClass,
   getLineId,
+  getShowLineNumbers,
 } from './utils';
 export type { Options, LineElement, CharsElement, Theme } from './types';
 
@@ -427,24 +427,29 @@ export function rehypePrettyCode(
           counterMap: new Map<string, number>(),
         };
 
+        // Detect `showLineNumbers` (optionally `showLineNumbers{N}`) as a
+        // standalone meta token, wherever it appears in the meta string.
+        // Occurrences inside a character-highlight annotation, which may
+        // legitimately span whitespace (`/foo showLineNumbers bar/`), are
+        // highlight text rather than an option, so the spans those
+        // annotations already claim are excluded.
+        const showLineNumbers = getShowLineNumbers(
+          meta,
+          (charsMatches ?? []).map((match) => {
+            const start = match.index ?? 0;
+            return [start, start + match[0].length];
+          }),
+        );
+
         // biome-ignore lint/complexity/noExcessiveCognitiveComplexity: <explanation>
         visit(codeTree, 'element', (element) => {
-          if (
-            element.tagName === 'code' &&
-            /srebmuNeniLwohs(?!(.*)(\/))/.test(reverseString(meta))
-          ) {
+          if (element.tagName === 'code' && showLineNumbers) {
             if (element.properties) {
               element.properties['data-line-numbers'] = '';
             }
 
-            const lineNumbersStartAtMatch = reverseString(meta).match(
-              /(?:\}(\d+){)?srebmuNeniLwohs(?!(.*)(\/))/,
-            );
-            const startNumberString = lineNumbersStartAtMatch?.[1];
-            if (startNumberString) {
-              const startAt = startNumberString
-                ? Number(reverseString(startNumberString)) - 1
-                : 0;
+            if (showLineNumbers.startAt !== null) {
+              const startAt = showLineNumbers.startAt - 1;
               lineNumbersMaxDigits = startAt;
               if (element.properties) {
                 element.properties.style = `counter-set: line ${startAt};`;

packages/core/src/utils.ts

@@ -111,6 +111,43 @@ export function replaceLineClass(element: Element) {
   }
 }
 
+// `showLineNumbers`, optionally `showLineNumbers{N}`, bounded by
+// whitespace or the ends of the meta string. Only lookahead is used, so
+// this stays parseable on Safari < 16.4, which lacks lookbehind.
+const showLineNumbersRegex = /(?:^|\s)(showLineNumbers(?:\{(\d+)\})?)(?=\s|$)/g;
+
+/**
+ * Finds the `showLineNumbers` option in a block meta string.
+ *
+ * Whitespace boundaries alone do not prove the token is an option: it can
+ * also be part of a delimited character-highlight annotation, as in
+ * `/foo showLineNumbers bar/`, where it is highlight text. `takenSpans`
+ * holds the `[start, end)` offsets already claimed by those annotations,
+ * and any match overlapping one of them is skipped.
+ *
+ * Returns `null` when the option is absent, otherwise `startAt` — the
+ * line number given by `showLineNumbers{N}`, or `null` for a bare token.
+ */
+export function getShowLineNumbers(
+  meta: string,
+  takenSpans: Array<[number, number]>,
+) {
+  for (const match of meta.matchAll(showLineNumbersRegex)) {
+    const token = match[1];
+    if (!token) continue;
+
+    // `match[0]` may carry a leading whitespace boundary character.
+    const start = (match.index ?? 0) + (match[0].length - token.length);
+    const end = start + token.length;
+
+    if (takenSpans.some(([from, to]) => start < to && end > from)) continue;
+
+    return { startAt: match[2] ? Number(match[2]) : null };
+  }
+
+  return null;
+}
+
 export function getLineId(lineNumber: number, meta: string) {
   const segments = meta.match(/\{[^}]+\}#[a-zA-Z0-9]+/g);
   if (!segments) return null;