karatelabs/karate · error · SyntaxError

Invalid regular expression: /

Error message

Invalid regular expression: /

What it means

After translating the JS regex source to Java syntax, the pattern is compiled with java.util.regex.Pattern. If Pattern.compile raises PatternSyntaxException, a SyntaxError 'Invalid regular expression: /pattern/ - <detail>' is thrown. This means the pattern is syntactically invalid even after Karate's JS→Java translation (or the translation itself does not cover the construct used).

Solutions

1. Read the PatternSyntaxException detail appended to the message for the exact position/problem
2. Fix the pattern: balance parens/brackets, correct quantifiers and escapes
3. Validate the pattern with a quick try/catch around new RegExp(pattern) before using it
4. Drop or rewrite JS-only constructs (e.g. use ([A-Za-z]) instead of \p{Alpha} when flags/translation do not support it)

Example fix

// before
new RegExp('a('); // unbalanced group -> PatternSyntaxException
// after
new RegExp('a(bc)?'); // balanced, valid pattern

function isValidPattern(p) {
  try { new RegExp(p); return true; } catch (e) { return false; }
}
if (!isValidPattern(pattern)) throw new Error('invalid regex: ' + pattern);

try {
  const re = new RegExp(pattern, flags);
} catch (e) {
  if (String(e.message).startsWith('Invalid regular expression')) {
    log.warn('bad regex pattern: ' + pattern + ' :: ' + e.message);
    return fallbackRegex;
  }
  throw e;
}

When it happens

Trigger: Patterns with unbalanced parentheses or brackets, invalid quantifiers like /*/, invalid escapes like \q, unterminated groups, or JS-only syntax (e.g. lookbehind forms, unicode property escapes \p{...} without the u flag) that translate to something Java rejects.

Common situations: Dynamically building patterns from user input; copying a JS regex using lookbehind or named-group syntax variants into an older translation path; forgetting the 'u' flag for \p{L}-style classes; hand-written patterns with an unmatched '('.

Related errors

• Invalid RegExp literal:
• extract() needs three arguments: text, regex, group
• extractAll() needs three arguments: text, regex, group
• String.prototype.replaceAll called with a non-global RegExp…
• String.prototype.matchAll called with a non-global RegExp…

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/aedfc53585783b75. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/JsRegex.java:97

            this.pattern = literalText.substring(1, lastSlashIndex);
            this.flags = lastSlashIndex < literalText.length() - 1
                    ? literalText.substring(lastSlashIndex + 1)
                    : "";
        } else {
            // string patterns without delimiters
            this.pattern = literalText;
            this.flags = "";
        }
        this.global = this.flags.contains("g");
        this.sticky = this.flags.contains("y");
        this.groupNames = extractGroupNames(this.pattern);
        int javaFlags = translateJsFlags(this.flags);
        try {
            // unescape js-specific regex syntax that differs from Java
            String javaPattern = translateJsRegexToJava(this.pattern);
            this.javaPattern = Pattern.compile(javaPattern, javaFlags);
        } catch (PatternSyntaxException e) {
            throw JsErrorException.syntaxError("Invalid regular expression: /" + pattern + "/ - " + e.getMessage());
        }
    }

    JsRegex(String pattern, String flags) {
        super(null, JsRegexPrototype.INSTANCE);
        this.pattern = pattern;
        this.flags = flags != null ? flags : "";
        this.global = this.flags.contains("g");
        this.sticky = this.flags.contains("y");
        this.groupNames = extractGroupNames(this.pattern);
        int javaFlags = translateJsFlags(this.flags);
        try {
            String javaPattern = translateJsRegexToJava(this.pattern);
            this.javaPattern = Pattern.compile(javaPattern, javaFlags);
        } catch (PatternSyntaxException e) {
            throw JsErrorException.syntaxError("Invalid regular expression: /" + pattern + "/ - " + e.getMessage());
        }
    }

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