What are common causes?

Escaping a character that does not need escaping in the current context. Using unsupported escapes (`\l`, `\u` without full unicode form) for the active flavor. Unescaped delimiter or backslash in a character class when it was meant to be literal.

Read the regex flavor docs for your runtime (e.g. JavaScript RegExp). Escape metacharacters only where required; use `\` to get a literal backslash when needed. Test incrementally in the regex tester until the pattern compiles.

Fix regex invalid escape

Invalid escape sequences in a regular expression cause a syntax error. Not all characters can be escaped, and some need escaping.

Each engine (JavaScript, PCRE, Python) allows different escapes. `\b` is word boundary in JS; `\k` may be invalid or mean something else depending on context.

Common causes

Escaping a character that does not need escaping in the current context.
Using unsupported escapes (`\l`, `\u` without full unicode form) for the active flavor.
Unescaped delimiter or backslash in a character class when it was meant to be literal.

How to fix

Read the regex flavor docs for your runtime (e.g. JavaScript RegExp).
Escape metacharacters only where required; use `\` to get a literal backslash when needed.
Test incrementally in the regex tester until the pattern compiles.

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