diff --git a/scripts/lib/shell-substitution.js b/scripts/lib/shell-substitution.js
index a2241770d..a5dec6fbb 100644
--- a/scripts/lib/shell-substitution.js
+++ b/scripts/lib/shell-substitution.js
@@ -94,6 +94,114 @@ function extractCommandSubstitutions(input, options = {}) {
   return [...iterateCommandSubstitutions(input, options)];
 }
 
+// --- Shared span readers ------------------------------------------------------
+// The group extractors below all need the same quote-aware skipping of
+// `$(...)`, backtick and `(...)` spans. Each reader takes the index of the
+// first character inside the span and returns where scanning should resume.
+
+function parenDelta(ch) {
+  if (ch === '(') return 1;
+  if (ch === ')') return -1;
+  return 0;
+}
+
+/**
+ * Toggle quote state for a quote character at the current position.
+ * Returns true when `ch` opened or closed a quote.
+ */
+function toggleQuote(ch, prev, quotes) {
+  if (ch === "'" && !quotes.inDouble && prev !== '\\') {
+    quotes.inSingle = !quotes.inSingle;
+    return true;
+  }
+  if (ch === '"' && !quotes.inSingle && prev !== '\\') {
+    quotes.inDouble = !quotes.inDouble;
+    return true;
+  }
+  return false;
+}
+
+/**
+ * Index just past the `)` that closes a paren span whose first inner character
+ * is at `start`, honoring quotes and escapes inside the span.
+ */
+function findParenSpanEnd(source, start) {
+  const quotes = { inSingle: false, inDouble: false };
+  let depth = 1;
+  let i = start;
+  while (i < source.length && depth > 0) {
+    const inner = source[i];
+    if (inner === '\\' && !quotes.inSingle) {
+      i += 2;
+      continue;
+    }
+    if (!toggleQuote(inner, source[i - 1], quotes) && !quotes.inSingle && !quotes.inDouble) {
+      depth += parenDelta(inner);
+    }
+    i += 1;
+  }
+  return i;
+}
+
+/**
+ * Index of the backtick that closes a span whose first inner character is at
+ * `start`, or `source.length` when the span is unterminated.
+ */
+function findBacktickEnd(source, start) {
+  let i = start;
+  while (i < source.length && source[i] !== '`') {
+    if (source[i] === '\\' && i + 1 < source.length) {
+      i += 2;
+      continue;
+    }
+    i += 1;
+  }
+  return i;
+}
+
+/**
+ * Top-level scan step shared by the group extractors: escapes, quote state,
+ * `$(...)` and backtick spans. Returns the last index consumed so a `for`
+ * loop's `i++` resumes after it, or -1 when the caller must inspect `source[i]`.
+ */
+function skipOuterSpan(source, i, quotes) {
+  const ch = source[i];
+  if (ch === '\\' && !quotes.inSingle) return i + 1;
+  if (toggleQuote(ch, source[i - 1], quotes)) return i;
+  if (quotes.inSingle || quotes.inDouble) return i;
+  if (ch === '$' && source[i + 1] === '(') return findParenSpanEnd(source, i + 2) - 1;
+  if (ch === '`') return findBacktickEnd(source, i + 1);
+  return -1;
+}
+
+/**
+ * Read a `(...)` subshell body starting just inside the opening paren. Returns
+ * the body text and the index of the closing paren (or where input ran out).
+ */
+function readSubshellBody(source, start) {
+  const quotes = { inSingle: false, inDouble: false };
+  let depth = 1;
+  let body = '';
+  let i = start;
+  while (i < source.length && depth > 0) {
+    const inner = source[i];
+    if (inner === '\\' && !quotes.inSingle) {
+      // Keep the escape and its operand together; a trailing backslash at the
+      // end of an unterminated span is appended once and the scan stops.
+      body += source.slice(i, i + 2);
+      i = Math.min(i + 2, source.length);
+      continue;
+    }
+    if (!toggleQuote(inner, source[i - 1], quotes) && !quotes.inSingle && !quotes.inDouble) {
+      depth += parenDelta(inner);
+      if (depth === 0) break;
+    }
+    body += inner;
+    i += 1;
+  }
+  return { body, end: i };
+}
+
 /**
  * Extract bodies of plain `(...)` subshell groups.
  *
@@ -114,118 +222,89 @@ function extractCommandSubstitutions(input, options = {}) {
 function extractSubshellGroups(input) {
   const source = String(input || '');
   const groups = [];
-  let inSingle = false;
-  let inDouble = false;
+  const quotes = { inSingle: false, inDouble: false };
 
   for (let i = 0; i < source.length; i++) {
-    const ch = source[i];
-    const prev = source[i - 1];
-
-    if (ch === '\\' && !inSingle) {
-      i += 1;
+    const skipped = skipOuterSpan(source, i, quotes);
+    if (skipped !== -1) {
+      i = skipped;
       continue;
     }
-
-    if (ch === "'" && !inDouble && prev !== '\\') {
-      inSingle = !inSingle;
-      continue;
-    }
-
-    if (ch === '"' && !inSingle && prev !== '\\') {
-      inDouble = !inDouble;
-      continue;
-    }
-
-    if (inSingle || inDouble) {
-      continue;
-    }
-
-    if (ch === '$' && source[i + 1] === '(') {
-      let depth = 1;
-      let skipInSingle = false;
-      let skipInDouble = false;
-      i += 2;
-      while (i < source.length && depth > 0) {
-        const inner = source[i];
-        const innerPrev = source[i - 1];
-        if (inner === '\\' && !skipInSingle) {
-          i += 2;
-          continue;
-        }
-        if (inner === "'" && !skipInDouble && innerPrev !== '\\') {
-          skipInSingle = !skipInSingle;
-        } else if (inner === '"' && !skipInSingle && innerPrev !== '\\') {
-          skipInDouble = !skipInDouble;
-        } else if (!skipInSingle && !skipInDouble) {
-          if (inner === '(') depth += 1;
-          else if (inner === ')') depth -= 1;
-        }
-        i += 1;
-      }
-      i -= 1;
-      continue;
-    }
-
-    if (ch === '`') {
-      i += 1;
-      while (i < source.length && source[i] !== '`') {
-        if (source[i] === '\\' && i + 1 < source.length) {
-          i += 2;
-          continue;
-        }
-        i += 1;
-      }
-      continue;
-    }
-
-    if (ch === '(') {
-      let depth = 1;
-      let body = '';
-      let bodyInSingle = false;
-      let bodyInDouble = false;
-      i += 1;
-      while (i < source.length && depth > 0) {
-        const inner = source[i];
-        const innerPrev = source[i - 1];
-        if (inner === '\\' && !bodyInSingle) {
-          body += inner;
-          if (i + 1 < source.length) {
-            body += source[i + 1];
-            i += 2;
-          } else {
-            // Trailing backslash at end of an unterminated span: advance past
-            // it so it is not appended a second time by the fallthrough below.
-            i += 1;
-          }
-          continue;
-        }
-        if (inner === "'" && !bodyInDouble && innerPrev !== '\\') {
-          bodyInSingle = !bodyInSingle;
-        } else if (inner === '"' && !bodyInSingle && innerPrev !== '\\') {
-          bodyInDouble = !bodyInDouble;
-        } else if (!bodyInSingle && !bodyInDouble) {
-          if (inner === '(') {
-            depth += 1;
-          } else if (inner === ')') {
-            depth -= 1;
-            if (depth === 0) {
-              break;
-            }
-          }
-        }
-        body += inner;
-        i += 1;
-      }
-      if (body.trim()) {
-        groups.push(body);
-        groups.push(...extractSubshellGroups(body));
-      }
+    if (source[i] !== '(') continue;
+    const { body, end } = readSubshellBody(source, i + 1);
+    i = end;
+    if (body.trim()) {
+      groups.push(body);
+      groups.push(...extractSubshellGroups(body));
     }
   }
 
   return groups;
 }
 
+/**
+ * Inside a brace body, `$(...)`, backtick and `(...)` spans are copied through
+ * verbatim so a `}` inside them cannot close the group. Returns the index just
+ * past the span, or -1 when `source[i]` does not open one.
+ */
+function findNestedSpanEnd(source, i) {
+  const ch = source[i];
+  if (ch === '$' && source[i + 1] === '(') return findParenSpanEnd(source, i + 2);
+  if (ch === '`') return Math.min(findBacktickEnd(source, i + 1) + 1, source.length);
+  if (ch === '(') return findParenSpanEnd(source, i + 1);
+  return -1;
+}
+
+// `{` is a reserved word only when followed by whitespace and preceded by the
+// line start, whitespace, or a shell operator (`;`, `|`, `&`, `(`).
+function opensBraceGroup(source, i) {
+  if (source[i] !== '{' || !/\s/.test(source[i + 1] || '')) return false;
+  return i === 0 || /[\s;|&(]/.test(source[i - 1]);
+}
+
+/**
+ * Read a `{ ...; }` brace body starting just after the opening brace. Returns
+ * the body text and the index of the closing brace (or where input ran out).
+ */
+function readBraceBody(source, start) {
+  const quotes = { inSingle: false, inDouble: false };
+  let depth = 1;
+  let body = '';
+  let i = start;
+  while (i < source.length && depth > 0) {
+    const inner = source[i];
+    const prev = source[i - 1];
+    if (inner === '\\' && !quotes.inSingle) {
+      body += source.slice(i, i + 2);
+      i = Math.min(i + 2, source.length);
+      continue;
+    }
+    if (toggleQuote(inner, prev, quotes) || quotes.inSingle || quotes.inDouble) {
+      body += inner;
+      i += 1;
+      continue;
+    }
+    const spanEnd = findNestedSpanEnd(source, i);
+    if (spanEnd !== -1) {
+      body += source.slice(i, spanEnd);
+      i = spanEnd;
+      continue;
+    }
+    if (inner === '{' && /\s/.test(source[i + 1] || '')) {
+      // Match the outer-scan boundary rule for nested `{` so tokens like
+      // `foo{` (no boundary, but followed by space via `foo{ bar`) cannot
+      // bump nested depth.
+      if (/[\s;|&(]/.test(prev)) depth += 1;
+    } else if (inner === '}' && (prev === ';' || /\s/.test(prev))) {
+      depth -= 1;
+      if (depth === 0) break;
+    }
+    body += inner;
+    i += 1;
+  }
+  return { body, end: i };
+}
+
 /**
  * Extract bodies of `{ ...; }` brace groups.
  *
@@ -250,228 +329,24 @@ function extractSubshellGroups(input) {
 function extractBraceGroups(input) {
   const source = String(input || '');
   const groups = [];
-  let inSingle = false;
-  let inDouble = false;
+  const quotes = { inSingle: false, inDouble: false };
 
   for (let i = 0; i < source.length; i++) {
-    const ch = source[i];
-    const prev = source[i - 1];
-
-    if (ch === '\\' && !inSingle) {
-      i += 1;
+    const skipped = skipOuterSpan(source, i, quotes);
+    if (skipped !== -1) {
+      i = skipped;
       continue;
     }
-
-    if (ch === "'" && !inDouble && prev !== '\\') {
-      inSingle = !inSingle;
+    if (source[i] === '(') {
+      i = findParenSpanEnd(source, i + 1) - 1;
       continue;
     }
-
-    if (ch === '"' && !inSingle && prev !== '\\') {
-      inDouble = !inDouble;
-      continue;
-    }
-
-    if (inSingle || inDouble) {
-      continue;
-    }
-
-    if (ch === '$' && source[i + 1] === '(') {
-      let depth = 1;
-      let skipInSingle = false;
-      let skipInDouble = false;
-      i += 2;
-      while (i < source.length && depth > 0) {
-        const inner = source[i];
-        const innerPrev = source[i - 1];
-        if (inner === '\\' && !skipInSingle) {
-          i += 2;
-          continue;
-        }
-        if (inner === "'" && !skipInDouble && innerPrev !== '\\') {
-          skipInSingle = !skipInSingle;
-        } else if (inner === '"' && !skipInSingle && innerPrev !== '\\') {
-          skipInDouble = !skipInDouble;
-        } else if (!skipInSingle && !skipInDouble) {
-          if (inner === '(') depth += 1;
-          else if (inner === ')') depth -= 1;
-        }
-        i += 1;
-      }
-      i -= 1;
-      continue;
-    }
-
-    if (ch === '`') {
-      i += 1;
-      while (i < source.length && source[i] !== '`') {
-        if (source[i] === '\\' && i + 1 < source.length) {
-          i += 2;
-          continue;
-        }
-        i += 1;
-      }
-      continue;
-    }
-
-    if (ch === '(') {
-      let depth = 1;
-      let skipInSingle = false;
-      let skipInDouble = false;
-      i += 1;
-      while (i < source.length && depth > 0) {
-        const inner = source[i];
-        const innerPrev = source[i - 1];
-        if (inner === '\\' && !skipInSingle) {
-          i += 2;
-          continue;
-        }
-        if (inner === "'" && !skipInDouble && innerPrev !== '\\') {
-          skipInSingle = !skipInSingle;
-        } else if (inner === '"' && !skipInSingle && innerPrev !== '\\') {
-          skipInDouble = !skipInDouble;
-        } else if (!skipInSingle && !skipInDouble) {
-          if (inner === '(') depth += 1;
-          else if (inner === ')') depth -= 1;
-        }
-        i += 1;
-      }
-      i -= 1;
-      continue;
-    }
-
-    if (ch === '{' && /\s/.test(source[i + 1] || '')) {
-      const prevIsBoundary = i === 0 || /[\s;|&(]/.test(prev);
-      if (!prevIsBoundary) continue;
-
-      let depth = 1;
-      let body = '';
-      let bodyInSingle = false;
-      let bodyInDouble = false;
-      i += 1;
-      while (i < source.length && depth > 0) {
-        const inner = source[i];
-        const innerPrev = source[i - 1];
-        if (inner === '\\' && !bodyInSingle) {
-          body += inner;
-          if (i + 1 < source.length) {
-            body += source[i + 1];
-            i += 2;
-          } else {
-            // Trailing backslash at end of an unterminated span: advance past
-            // it so it is not appended a second time by the fallthrough below.
-            i += 1;
-          }
-          continue;
-        }
-        if (inner === "'" && !bodyInDouble && innerPrev !== '\\') {
-          bodyInSingle = !bodyInSingle;
-          body += inner;
-          i += 1;
-          continue;
-        }
-        if (inner === '"' && !bodyInSingle && innerPrev !== '\\') {
-          bodyInDouble = !bodyInDouble;
-          body += inner;
-          i += 1;
-          continue;
-        }
-        if (bodyInSingle || bodyInDouble) {
-          body += inner;
-          i += 1;
-          continue;
-        }
-        // Skip $(...) spans — a quoted `}` or `}`-as-text inside a
-        // substitution body must not close the enclosing brace group.
-        if (inner === '$' && source[i + 1] === '(') {
-          body += inner + source[i + 1];
-          let subDepth = 1;
-          let subInSingle = false;
-          let subInDouble = false;
-          i += 2;
-          while (i < source.length && subDepth > 0) {
-            const c = source[i];
-            const p = source[i - 1];
-            body += c;
-            if (c === '\\' && !subInSingle && i + 1 < source.length) {
-              body += source[i + 1];
-              i += 2;
-              continue;
-            }
-            if (c === "'" && !subInDouble && p !== '\\') subInSingle = !subInSingle;
-            else if (c === '"' && !subInSingle && p !== '\\') subInDouble = !subInDouble;
-            else if (!subInSingle && !subInDouble) {
-              if (c === '(') subDepth += 1;
-              else if (c === ')') subDepth -= 1;
-            }
-            i += 1;
-          }
-          continue;
-        }
-        // Skip backtick spans for the same reason.
-        if (inner === '`') {
-          body += inner;
-          i += 1;
-          while (i < source.length && source[i] !== '`') {
-            if (source[i] === '\\' && i + 1 < source.length) {
-              body += source[i] + source[i + 1];
-              i += 2;
-              continue;
-            }
-            body += source[i];
-            i += 1;
-          }
-          if (i < source.length) {
-            body += source[i];
-            i += 1;
-          }
-          continue;
-        }
-        // Skip plain (...) subshell spans for the same reason.
-        if (inner === '(') {
-          body += inner;
-          let subDepth = 1;
-          let subInSingle = false;
-          let subInDouble = false;
-          i += 1;
-          while (i < source.length && subDepth > 0) {
-            const c = source[i];
-            const p = source[i - 1];
-            body += c;
-            if (c === '\\' && !subInSingle && i + 1 < source.length) {
-              body += source[i + 1];
-              i += 2;
-              continue;
-            }
-            if (c === "'" && !subInDouble && p !== '\\') subInSingle = !subInSingle;
-            else if (c === '"' && !subInSingle && p !== '\\') subInDouble = !subInDouble;
-            else if (!subInSingle && !subInDouble) {
-              if (c === '(') subDepth += 1;
-              else if (c === ')') subDepth -= 1;
-            }
-            i += 1;
-          }
-          continue;
-        }
-        if (inner === '{' && /\s/.test(source[i + 1] || '')) {
-          // Match the outer-scan boundary rule for nested `{` so
-          // tokens like `foo{` (no boundary, but followed by space
-          // via `foo{ bar`) cannot bump nested depth.
-          const nestedPrevIsBoundary = /[\s;|&(]/.test(innerPrev);
-          if (nestedPrevIsBoundary) depth += 1;
-        } else if (inner === '}' && (innerPrev === ';' || /\s/.test(innerPrev))) {
-          depth -= 1;
-          if (depth === 0) {
-            break;
-          }
-        }
-        body += inner;
-        i += 1;
-      }
-      if (body.trim()) {
-        groups.push(body);
-        groups.push(...extractBraceGroups(body));
-      }
+    if (!opensBraceGroup(source, i)) continue;
+    const { body, end } = readBraceBody(source, i + 1);
+    i = end;
+    if (body.trim()) {
+      groups.push(body);
+      groups.push(...extractBraceGroups(body));
     }
   }