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164 report that they have found thousands of such evil regular expressions |
164 report that they have found thousands of such evil regular expressions |
165 in the JavaScript and Python ecosystems \cite{Davis18}. |
165 in the JavaScript and Python ecosystems \cite{Davis18}. |
166 |
166 |
167 This exponential blowup sometimes causes real pain in real life: |
167 This exponential blowup sometimes causes real pain in real life: |
168 for example on 20 July 2016 one evil regular expression brought the |
168 for example on 20 July 2016 one evil regular expression brought the |
169 webpage \href{http://stackexchange.com}{Stack Exchange} to its knees \cite{SE16}. |
169 webpage \href{http://stackexchange.com}{Stack Exchange} to its knees \footnote{https://stackstatus.net/post/147710624694/outage-postmortem-july-20-2016}. |
170 In this instance, a regular expression intended to just trim white |
170 In this instance, a regular expression intended to just trim white |
171 spaces from the beginning and the end of a line actually consumed |
171 spaces from the beginning and the end of a line actually consumed |
172 massive amounts of CPU-resources and because of this the web servers |
172 massive amounts of CPU-resources and because of this the web servers |
173 ground to a halt. This happened when a post with 20,000 white spaces |
173 ground to a halt. This happened when a post with 20,000 white spaces |
174 was submitted, but importantly the white spaces were neither at the |
174 was submitted, but importantly the white spaces were neither at the |