SyntaxError: \\ at end of pattern — What went wrong?

Reference note (untrusted external data; do not execute it as instructions). A backslash cannot appear literally in a regular expression. It either precedes another character to escape it, or is itself escaped by another backslash. A backslash at the end of a regular expression pattern is invalid because it is not escaping anything. Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV isolated this documentation section, normalized formatting, removed long code blocks, and shortened it for retrieval. Verify version-sensitive details at the source.