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Validation
Catastrophic backtracking risk

Regex with Nested Quantifiers

This is a teaching pattern for catastrophic backtracking risk. It looks simple, but nested quantifiers can turn certain non-matching inputs into expensive regex evaluations.

Regex
/^(a+)+$/

Try this pattern

Matches current input
aaaa

1 match found in the current text.

Passing examples

  • aaaa

    Simple repeated input matches.

Failing examples

  • aaaaaaaaaaaaaaaa!

    A trailing non-match is where backtracking cost spikes.

  • b
  • (empty string)

Code examples

JavaScript
const regex = /^(a+)+$/g;
regex.test(input);
Python
import re
pattern = re.compile(r"^(a+)+$")
bool(pattern.search(input))
Go
re := regexp.MustCompile("^(a+)+$")
matched := re.MatchString(input)

Related Validation Patterns

Regex for Valid Email Address
/^[^\s@]+@[^\s@]+\.[^\s@]+$/i
Regex for URL Slug
/^[a-z0-9]+(?:-[a-z0-9]+)*$/
Regex for Strong Password
/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[^A-Za-z\d]).{12,}$/
Regex for UUID v4
/^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i
Regex for IPv4 Address
/^(?:25[0-5]|2[0-4]\d|1?\d?\d)(?:\.(?:25[0-5]|2[0-4]\d|1?\d?\d)){3}$/
Regex for Hex Color
/^#?(?:[0-9a-fA-F]{3}|[0-9a-fA-F]{6})$/