query-params.htaccess

# Query Parameter Handling (Apache / .htaccess)
#
# What this does:
#   Demonstrates three query-string strategies during a redirect:
#   preserve the original params, strip them, or route based on a
#   specific parameter's value. Requires mod_rewrite because decisions
#   depend on %{QUERY_STRING}.
#
# When to use it:
#   Preserve - keep UTM/tracking params through a URL migration.
#   Strip    - clean dirty legacy URLs down to a canonical target.
#   Route    - map old ?id=x style URLs to new clean paths.
#
# Notes:
#   By default Apache APPENDS the incoming query string to the target.
#   A trailing "?" on the substitution DROPS it. The [QSA] flag merges
#   new params with the originals.

RewriteEngine On

# 1) PRESERVE (default behavior). /old?utm_source=x -> /new?utm_source=x
RewriteRule ^old$ /new [R=301,L]

# 2) STRIP the query string. The trailing "?" discards incoming params.
#    /messy?sid=abc&ref=y -> /clean
RewriteRule ^messy$ /clean? [R=301,L]

# 3) ROUTE on a specific parameter value.
#    /article?id=42 -> /articles/42
RewriteCond %{QUERY_STRING} (?:^|&)id=([^&]+)
RewriteRule ^article$ /articles/%1? [R=301,L]

# Example:
#   Before: https://example.com/old?utm_source=newsletter
#   After:  https://example.com/new?utm_source=newsletter   (HTTP 301)
#
# Gotchas:
#   - Without a trailing "?", the original query string is re-appended to
#     the target - the #2 "strip" case needs that "?" to work.
#   - %1 comes from the RewriteCond capture (the id value); $1 would come
#     from the RewriteRule pattern. Do not mix them up.
#   - The RewriteRule pattern matches the path WITHOUT a leading slash in
#     .htaccess context (per-directory), hence ^old$ not ^/old$.
#   - Add the [QSA] flag if the target already has its own query string
#     and you want to merge rather than replace.