Why this code stuck node.js - Bug on Javascript?

Question

I'm trying to run this regex but it stuck my console. Why?

var str = "Шедевры православной музыки - 20 золотых православных песен";
str.match(/^(([\u00C0-\u1FFF\u2C00-\uD7FF]+[^a-z\u00C0-\u1FFF\u2C00-\uD7FF]*)+) [a-z]+[^\u00C0-\u1FFF\u2C00-\uD7FF]*$/i);

Answer 1

Your regex causes catastrophic backtracking (see a demo of your regex here) due to (([\u00C0-\u1FFF\u2C00-\uD7FF]+[^a-z\u00C0-\u1FFF\u2C00-\uD7FF]*)+) part. As [^a-z\u00C0-\u1FFF\u2C00-\uD7FF]* can match zero characters, you basically have a classical (a+)+-like pattern (cf: ([\u00C0-\u1FFF\u2C00-\uD7FF]+)+) that causes backtracking issue.

To get rid of it, you need to make sure the subpatterns are compulsory inside the grouping, and apply a * quantifier to the whole grouping:

^([\u00C0-\u1FFF\u2C00-\uD7FF]+(?:[^a-z\u00C0-\u1FFF\u2C00-\uD7FF]+[\u00C0-\u1‌​FFF\u2C00-\uD7FF]+)*) [a-z]+[^\u00C0-\u1FFF\u2C00-\uD7FF]*$

See regex demo

Here, [\u00C0-\u1FFF\u2C00-\uD7FF]+(?:[^a-z\u00C0-\u1FFF\u2C00-\uD7FF]+[\u00C0-\u1‌​FFF\u2C00-\uD7FF]+)* matches:

• [\u00C0-\u1FFF\u2C00-\uD7FF]+ - one or more character from [\u00C0-\u1FFF\u2C00-\uD7FF] ranges
• (?:[^a-z\u00C0-\u1FFF\u2C00-\uD7FF]+[\u00C0-\u1‌​FFF\u2C00-\uD7FF]+)* - zero or more sequences of:
  • [^a-z\u00C0-\u1FFF\u2C00-\uD7FF]+ - one or more characters other than those from the a-z\u00C0-\u1FFF\u2C00-\uD7FF ranges
  • [\u00C0-\u1‌​FFF\u2C00-\uD7FF]+ - one or more characters from the \u00C0-\u1‌​FFF\u2C00-\uD7FF ranges.