Why does this regex take a long time to execute?

Question

I found out, that for example this line has a very very long execution time:

System.out.println(
        \".. .. .. .. .. .. .. .. ..  .. .. .. .. .. .. .. .         


        

Answer 1

When pattern x is made optional - using ? or * quantifiers (or {0,}) - engine has two paths to approach according to the nature of quantifier being used:

• Consumes then backtracks for other patterns (case of greediness i.e. .*, .?)
• First doesn't consume and looks immediately for other patterns (case of laziness .*?)

Someone probably is not aware about regular expressions or doesn't care about performance and throws .* wherever he needs a match somewhere in string and engines are so fast in taking steps back and forth that nothing seems weird or slow unless a pattern can not be found.

Time complexity starts at O(n) and continues with O(n^2b) where b is level of nesting quantifiers. So on failure number of steps an engine takes is HUGE.

To avoid such situations someone needs to consider some guiding principles:

• Specifying boundaries. If pattern should stop somewhere before digits do not do .*. Instead do \D*.

• Use conditions. You can check if pattern / letter x exists before running a whole match using a lookahead ^(?=[^x]*x). This leads to an early failure.

• Use possessive quantifiers or atomic groups (if available). These two avoid backtracks. Sometimes you do not need backtracks.

• Do not do (.*)+ or similar patterns. Instead reconsider your requirements or at least use atomic groups (?>.*)+.

Your own Regular Expression isn't an exception. It suffers from much greediness and optional matches and needs a time to be restudied.