Test if a string is regex

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The simplest way to test if a string is a regex is:

if( preg_match("/^\/.+\/[a-z]*$/i",$regex))

This will tell you if a string has a good chance of being intended to be as a regex. However there are many string that would pass that check but fail being a regex. Unescaped slashes in the middle, unknown modifiers at the end, mismatched parentheses etc. could all cause problems.

The reason preg_last_error returned 0 is because the “invalid regex” is not:

  • PREG_INTERNAL_ERROR (an internal error)
  • PREG_BACKTRACK_LIMIT_ERROR (excessively forcing backtracking)
  • PREG_RECURSION_LIMIT_ERROR (excessively recursing)
  • PREG_BAD_UTF8_ERROR (badly formatted UTF-8)
  • PREG_BAD_UTF8_OFFSET_ERROR (offset to the middle of a UTF-8 character)

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