grep: Literal Fallback

When grep receives a pattern that fails regex compilation, it now checks whether the input looks like intended regex syntax before returning an error.

How It Works

If the pattern fails to compile and does not look like intentional regex (no alternation |, wildcards .*/.+, anchors ^/$, escape sequences \w/\d/\b, or balanced grouping (...)), the tool falls back to a literal text search by escaping all metacharacters automatically.

// User types this — unescaped `(` makes it invalid regex,
// but there is no closing `)` so it looks like plain text:
pattern: "if (x > 0"

// codescout escapes it to: \Qif \(x > 0\E  (effectively)
// and searches for the literal string

The response includes two extra fields to signal the fallback:

{
  "matches": [...],
  "total": 2,
  "mode": "literal_fallback",
  "reason": "pattern was not valid regex — searched as literal text"
}

The compact format output is prefixed with [literal fallback] so the mode is visible at a glance.

When the Error Is Preserved

If the broken pattern contains regex-like syntax — alternation (foo|bar), quantified wildcards (fn.*call), escape sequences (\w+), etc. — the original RecoverableError is returned. This avoids silently misinterpreting a malformed regex as a literal string.

// "(foo|bar" — has alternation, so is_regex_like = true
// → RecoverableError: "invalid regex: unclosed group"
pattern: "(foo|bar"

Compact Output

[literal fallback] 3 matches
src/lib.rs:42  if (x > 0) {
src/lib.rs:87  if (x > 0) {
src/lib.rs:103 if (x > 0) {