“This Regular Expression Has Been Replaced With a Substring Function”
The direct cause was a malformed post that caused one of our regular expressions to consume high CPU on our web servers. The post was in the homepage list, and that caused the expensive regular expression to be called on each home page view. This caused the home page to stop responding fast enough. Since the home page is what our load balancer uses for the health check, the entire site became unavailable since the load balancer took the servers out of rotation.
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The regular expression was: ^[\s\u200c]+|[\s\u200c]+$ Which is intended to trim unicode space from start and end of a line. A simplified version of the Regex that exposes the same issue would be \s+$ which to a human looks easy (“all the spaces at the end of the string”), but which means quite some work for a simple backtracking Regex engine.
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This is not classic catastrophic backtracking (talk on backtracking) (performance is O(n²), not exponential, in length), but it was enough. This regular expression has been replaced with a substring function.
