Is there a regular expression to detect a valid regular expression?

No, you generally cannot write a single, all-purpose regular expression that validates every possible “valid regex.”** Different regex engines have different syntax rules, and even within one engine, the grammar for “what constitutes a valid regex” can be quite complex—beyond the capabilities of a single pattern.

Below is some additional context about why that is, and what you can do if you need to check whether a string is a valid regex:

1. Why a Single “Universal Regex” Won’t Work

1. Different Engines, Different Syntax

   • JavaScript, PCRE (Perl-Compatible Regular Expressions), Python, .NET, and other engines each have unique features (lookbehind, named captures, modifiers, etc.).
   • A pattern that’s valid in one engine might not be valid in another.

2. Complex Grammar

   • A regex itself can contain nested groups, lookarounds, alternation, character classes, and more. This is effectively a context-free or more complex grammar.
   • A single “flat” pattern (regular language) typically can’t parse all possibilities of a more complex grammar.

3. Escaping & Edge Cases

   • Even seemingly small details—like how you escape backslashes, quotes, or bracket pairs—can vary by engine.
   • Checking for balanced parentheses or brackets might be simple at first, but once you account for nesting and special constructs, it becomes a much bigger problem.

2. What You Can Do

1. Try to Compile / Catch Errors

   • If you’re in a specific environment (e.g., Java, JavaScript, Python), a practical approach is to attempt to compile the user’s string as a regex. If it fails, you know it’s invalid.
   • For example (in JavaScript):

     function isValidRegex(str) {
       try {
         new RegExp(str);
         return true;
       } catch (e) {
         return false;
       }
     }

   • This method respects the actual engine’s rules without you having to manually parse or guess them.

2. Use a Dedicated Parser

   • If you need to parse or analyze patterns in more detail (e.g., rewriting them, checking for certain features), you can look for a “regex parser” library that specifically handles the grammar of your target engine.
   • These parsers are typically not just a single “regex to validate a regex,” but rather a small parser that builds an abstract syntax tree (AST) and can handle edge cases.

3. Restrict the Scope

   • If you only need to validate a simpler subset of regex features (for example, no nested groups, or no lookbehind), you might craft a pattern or parser that checks that subset. But this is not the same as “validate any possible real-world regex.”

3. Illustration via Example

If someone tries something like:

(abc)+  # Valid in many regex engines

No single universal “regex to validate that regex” would be guaranteed to say it’s valid in all engines, because some might interpret whitespace differently or might need explicit flags. Then consider something like:

(?<=\d)abc

• This uses a lookbehind, which not all engines support. So any “universal validator” that sees ?<= could erroneously mark it as valid or invalid, depending on which syntax it has been built to expect.

4. Conclusion

• A single regex that universally detects “valid regex syntax” for all engines does not exist.
• Even within one engine’s feature set, writing a single flat regex to parse all possible valid constructs is extremely difficult (or impossible) due to the grammar’s complexity.
• Best Practice: If your goal is practical validity for your environment (like JavaScript), simply try to compile the string as a regex (new RegExp(str)) inside a try/catch. If it fails, it’s invalid. If it succeeds, you know it’s valid for that engine’s grammar.

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Bottom line: There’s no single universal “regex to validate regexes.” Use compilation or a dedicated parser in your actual target engine for a more reliable validation approach.