RewriteRule

RewriteRule is a directive of the Apache HTTP Server's mod_rewrite module that defines rules for rewriting requested URLs. It enables server-side URL transformation, supporting internal rewrites, redirects, and complex routing strategies used by many web applications to present clean or stable URLs.

Syntax and usage: A rule has the form RewriteRule Pattern Substitution [Flags]. Pattern is a regular expression matched against the URL path being processed. Substitution is the target URL or path to serve or redirect to. Flags modify behavior, including L for last rule, R for external redirects, and QSA to preserve the original query string.

Context and matching rules: Rules may be placed in server or virtual host configuration, directory sections, or .htaccess files. In per-directory context, the Pattern matches the URL-path relative to the directory, and the initial path prefix is stripped before evaluation. RewriteCond directives can be used to enable a RewriteRule only when a condition is met.

Processing and effects: When a request is processed, mod_rewrite evaluates rules in order. If a rule matches, its Substitution is applied. The request may be rewritten internally or redirected to a new URL depending on flags. The L flag halts further rule processing for the current pass. Multiple passes can occur if a substitution changes the URL.

Common use cases include creating clean URLs for content management systems, implementing front controllers for web frameworks, canonical redirects, and basic access control. Example: RewriteRule ^articles/([0-9]+)/(.*)$ article.php?id=$1&title=$2 [L,QSA]

Security and maintenance: Misconfigured rules can cause redirect loops or performance issues. Testing configurations and documenting rules help maintain predictable behavior; prefer explicit patterns, and avoid broad wildcards that can affect large portions of the site. See also mod_rewrite and RewriteCond.