Skip to content

Regex for hashtags tree takes too long to execute

In our app we have topics which user can create, and each topic must have it’s own hashtag (or hashtags hierarchy). We have this kinda Regex for validation:

const REGEX_HASHTAG = /^(#[w]?((/?)([a-z0-9]+)+)+)(,s{0,1}#[a-z0-9]?((/?)([a-z0-9]+)+)+)*$/g;

What i need is for user to be able to create hashtags which have structure like this:

  1. (#) symbol
  2. Text in lowercase
  3. Optional slash (/) followed by lowercase text to create hierarchy

And also users can put comma (and optional whitespace) followed by new hashtag or hashtag hierarchy. When i put too many letters and slash at the end Regex stops working, takes too long to execute. What am i doing wrong?


There are quite a few nested quantifiers and optional parts, which can cause catastrophic backtracking when there is no match.

You could write the pattern as

  • ^ Start of string
  • #[a-z0-9]+ Match # and 1+ repetitions of the listed characters in the character class
  • (?:/[a-z0-9]+)* Optionally repeat the / and the same character class
  • (?: Non capture group
    • ,s* Match a comma and optional whitespace chars
    • #[a-z0-9]+(?:/[a-z0-9]+)* The same pattern as in the first part
  • )* Close the non capture group and optionally repeat it
  • $ End of string

Regex demo