# Single-character super-linear RegExps

<sup><sub>what a name...</sub></sup>

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A library to find JS RegExp with super-linear worst-case time complexity for attack strings that repeat a single character.

The static analysis method implemented by this library focuses on finding attack string tuples where a single character is repeated. This major limitation allows the library to be fast while also offering decent support for backreferences and [assertions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions/Assertions).

This library is not intended as a full static analysis to guard against super-linear worst-case time complexity. It is meant to be as a supplementary analysis on top of existing general analysis methods that don't (or don't fully) support advanced regex features, or as a lightweight analysis on top of existing full (but heavyweight) analysis methods. Libraries that provide such general or near-full analysis are known as [recheck](https://github.com/MakeNowJust-Labo/recheck) and [vuln-regex-detector](https://github.com/davisjam/vuln-regex-detector). You may consider using these libraries as well.


## Usage

This library exports only a single function, `analyse`, which takes a RegExp literal and returns a list of reports that show the quantifiers causing super-linear worst-case time complexity.

### Documentation

For more information on the exact inputs and outputs of each function, see the full API documentation.

- [Latest release](https://rundevelopment.github.io/scslre/docs/latest/)
- [Development](https://rundevelopment.github.io/scslre/docs/dev/)


## Limitations

### Analysis

This library is implemented using a very limited static analysis method that can only find attack strings where a single character is repeated. Attack strings are generated from a tuple _(x,y,z)_ such that every string _s = xy<sup>n</sup>z_ (or `x + y.repeat(n) + z` for JS folks) takes _O(n<sup>p</sup>)_ or _O(2<sup>n</sup>)_ many steps to reject, p>1. This analysis method can only find tuples where _y_ is a single character. E.g. the polynomial backtracking in `/^(ab)*(ab)*$/` for _(x,y,z) = ("", "ab", "c")_ cannot be detected by this library because _y_ is not a single character.

However, this limitation allows the static analysis method to be quick and to provide good (but not perfect) support for backreferences and assertions (e.g. `\b`, `(?<!ba+)`).

### False negatives

The analysis method primarily searches for polynomial backtracking. Finds of exponential backtracking are only a byproduct. Because of this, not all causes of super-linear worst-case time complexity are found.

### False positives

This library doesn't actually search for the whole tuple _(x,y,z)_; it only searches for _y_ and assumes that adequate values for _x_ and _z_ can be found. A single-character approximation of the suffix _z_ will be computed and accounted for but false positives are still possible.


## Reports

There are 3 different types of reports that each indicate a different type of cause for the super-linear worst-case time complexity. All are explained in the documentation of their types.

### Exponential backtracking

While most reports show polynomial backtracking, some report exponential backtracking. Exponential backtracking is a lot more dangerous and can easily be exploited for [ReDoS attacks](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS).

While other reports may be dismissed, __all reports of exponential backtracking must be fixed__.

All reports with `exponential: true` report exponential backtracking.