What causes "The first regexp in the regexp pair, "%s", is not a valid regexp." in phacility/phabricator?

Fix the FIRST element of the JSON array so it is a valid PCRE pattern with delimiters, e.g. "@^title$@".

phacility/phabricator · error · HeraldInvalidConditionException

The first regexp in the regexp pair, "%s", is not a valid re

Error message

The first regexp in the regexp pair, "%s", is not a valid regexp.

What it means

Thrown by HeraldAdapter::willSaveCondition() for CONDITION_REGEXP_PAIR after the JSON array was decoded and had exactly two elements. The first element is shifted off as the key regexp and smoke-tested with @preg_match($key_regexp, ''); a boolean false return (invalid PCRE pattern) raises this error, naming the first (key) regexp specifically.

Source

Thrown at src/applications/herald/adapter/HeraldAdapter.php:655

              'The regular expression pair "%s" is not valid JSON. Enter a '.
              'valid JSON array with two elements.',
              $condition_value));
        }

        if (count($json) != 2) {
          throw new HeraldInvalidConditionException(
            pht(
              'The regular expression pair "%s" must have exactly two '.
              'elements.',
              $condition_value));
        }

        $key_regexp = array_shift($json);
        $val_regexp = array_shift($json);

        $key_ok = @preg_match($key_regexp, '');
        if ($key_ok === false) {
          throw new HeraldInvalidConditionException(
            pht(
              'The first regexp in the regexp pair, "%s", is not a valid '.
              'regexp.',
              $key_regexp));
        }

        $val_ok = @preg_match($val_regexp, '');
        if ($val_ok === false) {
          throw new HeraldInvalidConditionException(
            pht(
              'The second regexp in the regexp pair, "%s", is not a valid '.
              'regexp.',
              $val_regexp));
        }
        break;
      case self::CONDITION_CONTAINS:
      case self::CONDITION_NOT_CONTAINS:
      case self::CONDITION_IS:

View on GitHub (pinned to 5720a38cfe)

Solutions

Fix the FIRST element of the JSON array so it is a valid PCRE pattern with delimiters, e.g. "@^title$@".
Escape or change the delimiter if the pattern itself contains it.
Test the key pattern standalone: php -r 'var_dump(@preg_match("@^title$@", ""));' — it must not print bool(false).

Example fix

// before
["^title$", "@urgent@"]

// after
["@^title$@", "@urgent@"]

Defensive patterns

Strategy: validation

Validate before calling

list($key_re, $val_re) = phutil_json_decode($value);
if (@preg_match($key_re, '') === false) {
  return pht('First regexp of the pair is invalid; add delimiters.');
}

Type guard

function isValidRegexpPairKey($pair_json) {
  $json = phutil_json_decode($pair_json);
  return @preg_match($json[0], '') !== false;
}

Try / catch

try {
  $editor->save();
} catch (HeraldInvalidConditionException $ex) {
  // 'first regexp ... is not valid' -> fix element 0 only, then resubmit
}

Prevention

Validate BOTH pair elements with @preg_match($re, '') in one pre-save pass; errors 823 and 824 surface one at a time.
Use a shared regexp sanitizer so key and value patterns are formatted identically.

When it happens

Trigger: Saving a regexp pair like ["title.*", "@value@"] — the first element lacks delimiters, has an unbalanced delimiter, an invalid modifier, or a PCRE syntax error. The second element is validated separately, so this error points only at element zero.

Common situations: Users add delimiters to one element of the pair but not the other, or paste a bare grep-style pattern as the key matcher while correctly formatting the value matcher.

Related errors

AI-assisted analysis of phacility/phabricator@5720a38cfe (2026-08-21). Data as JSON: /api/errors/261b1ea72ae4b431. Report an issue: GitHub.