If you are familiar with regular expressions and use repeated expressions in the 0-width assertion *, an error will be reported. Solution

If you are familiar with regular expressions and use repeated expressions in 0-width assertions *, an error will be reported. PHPcodepreg_match ('#(? & Lt; = a *) word # ', 'preword', $ mt); preg_match ('#(? & Lt; = & lt; (\ w +) & gt ;).*(? = & Lt; \/\ 1 & gt;) # ', if 'P is familiar with regular expressions, an error is returned when the 0-width assertion is repeated *
Let's take a look at the two.

PHP code

  Preg_match ('#(? <= A *) word # ', 'pre word', $ mt); preg_match ('#(? <= <(\ W +)> ).*(? = <\/\ 1>) # ', 'prehi! Word', $ mt );

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Http://perldoc.perl.org/perlretut.html#Looking-ahead-and-looking-behind
This is a perl regular expression document and I think it should be applicable to the php regular expression engine.
The lookahead assertion is denoted (? = Regexp) and the lookbehind assertion is denoted (? <= Fixed-regexp)
The zero-width assertion is also called the loop View. the above meaning is that the reverse-order loop view can only contain a fixed-length subexpression, so it cannot appear *? + These quantifiers are only available for sequential view.