Get the latest download link programmatically

Update:
Solution that determines and downloads the latest stable version via http://nginx.org/en/download.html (http://nginx.org/download/, used in the original solution below, does not distinguish between stable and mainline versions) - works on both Linux and OSX:

# Determine the latest stable version's download URL, assumed to be 
# the first `/download/nginx-*.tar.gz`-like link following the header 
# "Stable version".
latestVer=$(curl -s 'http://nginx.org/en/download.html' | 
   sed 's/</\'$'\n''</g' | sed -n '/>Stable version$/,$ p' | 
   egrep -m1 -o '/download/nginx-.+\.tar\.gz')

# Download.
curl "http://nginx.org${latestVer}" > nginx-latest.tar.gz

Note: This relies on specifics of the HTML structure of page http://nginx.org/en/download.html, which is not the most robust solution.

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Original solution that determines the latest version via http://nginx.org/download/, where no distinction is made between stable and mainline versions:

On Linux, try:

 # Determine latest version:
 latestVer=$(curl 'http://nginx.org/download/' | 
   grep -oP 'href="nginx-\K[0-9]+\.[0-9]+\.[0-9]+' | 
   sort -t. -rn -k1,1 -k2,2 -k3,3 | head -1)

 # Download latest version:
 curl "http://nginx.org/download/nginx-${latestVer}.tar.gz" > nginx-latest.tar.gz

This does NOT rely on a specific listing order at http://nginx.org/download/; instead, version numbers are extracted and sorted appropriately.

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On OSX - where grep doesn't support -P and \K for dropping the left part of a match is not available, try:

# Determine latest version:
latestVer=$(curl 'http://nginx.org/download/' | 
 egrep -o 'href="nginx-[0-9]+\.[0-9]+\.[0-9]+' | sed 's/^href="nginx-//' |
 sort -t. -rn -k1,1 -k2,2 -k3,3 | head -1)

# Download latest version:
curl "http://nginx.org/download/nginx-${latestVer}.tar.gz" > nginx-latest.tar.gz

This will determine the latest stable version:

$ lynx -dump http://nginx.org/en/download.html \
    | awk '/Stable/{t=1}t&&/nginx-/{$0=$2;sub(/.+\]/,"");print;exit}'
nginx-1.4.7

Use this result to assemble a proper download URL and use wget/curl to download.

This will determine the download URL of the most recent release:

$ lynx -dump http://nginx.org/download/ | awk '/nginx-.*\.zip$/{url=$2}END{print url}'
http://nginx.org/download/nginx-1.5.9.zip

This relies on their webserver sorting the directory listing by date (which it currently does).