> ## 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. # Write rules > Semgrep uses rules, which encapsulate pattern matching logic and data flow analysis, to scan your code for security issues, style violations, bugs, and more. In addition to rules available to you in the Semgrep Registry, you can write custom rules to determine what Semgrep detects in your repositories. You can write rules that: * Automate code review comments. * Identify secure coding violations. * Scan configuration files. See more use cases in [Rule ideas](/writing-rules/rule-ideas). ## Get started For an introduction to writing Semgrep rules, use the interactive, example-based [Semgrep rule tutorial](https://semgrep.dev/learn). You can write rules in your terminal and run them with the Semgrep command line tool, or you can write and test using the [Semgrep Editor](https://semgrep.dev/editor). For example, the following sample rule detects the use of `is` when comparing Python strings. `is` checks reference equality, not value equality, and can exhibit nondeterministic behavior.