Rewrite rule to transform part of the path to a query string key/value pair

apache_rewrite.ini

RewriteEngine On

#
# This rewrite rule will change a URI from something like :
# http://foo.com/corpus/59/d0/400_59d022a8dcc082473fac85dbb219e724SongStream-ParadeofLights-ServerSide-640x765.png
# Notice this part of the URI ^^^ -- that becomes a query parameter...
# i.e, into http://foo.com/corpus/59/d0/59d022a8dcc082473fac85dbb219e724SongStream-ParadeofLights-ServerSide-640x765.png?width=400
# 

#
# This should be placed in the 'root' of the httpd.conf file. 
# This is done so that all the VirtualHost, <Directory> level rewrites/aliases will take over.
#

# If the query string is empty..
RewriteCond %{QUERY_STRING} ^$ [OR]

# OR doesn't contain an explicit width= parameter in its query string..
RewriteCond %{QUERY_STRING} !width=

# Make sure that the request filename is not present
RewriteCond %{REQUEST_FILENAME} !-f

# Match the request URI against the following patterns:
# Match Group anything ahead of the final / (the actual file name) to %1
# Match the chars following the / to a integer number (this will become the 'width' query param to %2
# Ensure that there is a '_' char after the number
# Match and group the everything after the "_", but before the "." (the filename, sans extension) to %3
# Ensure that there is a '.' char after the filename
# Ensure that the chars after the '.' is a allowed extension, and group that into %4
# The NC flag makes sure that the match ignores case
RewriteCond %{REQUEST_URI} ^/(.+)/(\d+)_(.+)\.(png|jpg|jpeg|gif)$ [NC]

#
# Now, reconstruct the URI into the new format. Notice the QSA flag, which appends any other query
# parameters that might be present. We do want them to be passed on. Apache takes care of making sure there
# is only one '?' in the URI and all that.
#

#
# Also, notice the "PT" flag, which means 'passthrough', which allows other rules to cascade the URI.
# This lets things like aliases, which might be defined at the directory level, to work.
#
RewriteRule (.*) /%1/%3.%4?width=%2 [PT,QSA]