> ## Documentation Index
> Fetch the complete documentation index at: https://docs.semgrep.dev/llms.txt
> Use this file to discover all available pages before exploring further.
# Metavariable analysis
Semgrep developed metavariable analysis to support several metavariable inspection techniques that are difficult to express with existing rules, but have "simple" binary classifier behavior. Currently, this syntax supports two analyzers: `redos` and `entropy`.
## ReDoS
```yaml theme={null}
metavariable-analysis:
analyzer: redos
metavariable: $VARIABLE
```
Poorly constructed regular expressions that exhibit exponential runtime when fed specifically crafted inputs can cause RegEx denial of service. The `redos` analyzer uses known RegEx anti-patterns to determine if the target expression is potentially vulnerable to catastrophic backtracking.
## Entropy
```yaml theme={null}
metavariable-analysis:
analyzer: entropy
metavariable: $VARIABLE
```
Entropy is a common approach for detecting secret strings. Many existing tools utilize a combination of entropy calculations and regular expressions (RegEx) for secret detection. This analyzer returns `true` if a metavariable has high entropy, or randomness, relative to the English language.