Regular expression matching can be simple and fast,using finite automata-based techniques that have been known for decades. In contrast,Perl,PCRE,Python,Ruby,Java,and many other languages have regular expression implementations based on recursive backtracking that are simple but can be excruciatingly slow. With the exception of backreferences,the features provided by the slow backtracking implementations can be provided by the automata-based implementations at dramatically faster,more consistent speeds.

大多数情况下，归结为“常规”表达式中的非正则特征的泛滥，例如反向引用，以及大多数程序员的(继续)无知，对于不包含这样的特征的正则表达式(其中许多是更好的替代) 。

While writing the text editor sam in the early 1980s,Rob Pike wrote a new regular expression implementation,which Dave Presotto extracted into a library that appeared in the Eighth Edition. Pike’s implementation incorporated submatch tracking into an efficient NFA simulation but,like the rest of the Eighth Edition source,was not widely distributed. Pike himself did not realize that his technique was anything new. Henry Spencer reimplemented the Eighth Edition library interface from scratch,but using backtracking,and released his implementation into the public domain. It became very widely used,eventually serving as the basis for the slow regular expression implementations mentioned earlier: Perl,and so on. (In his defense,Spencer knew the routines could be slow,and he didn’t know that a more efficient algorithm existed. He even warned in the documentation,“Many users have found the speed perfectly adequate,although replacing the insides of egrep with this code would be a mistake.”) Pike’s regular expression implementation,extended to support Unicode,was made freely available with sam in late 1992,but the particularly efficient regular expression search algorithm went unnoticed.