Most efficient regular expression for Nginx location

Question

What is the most efficient way to define a location directive which matches something like

location = /[0-9a-zA-Z_-]{1,6} { content_by_lua_file ....}

In other words a URI which matches a string from 1 to 6 characters with "-", "_", digits and letters.

Or is it faster to check string length within my LUA code, which will generate the output by using a location directive like

location  / {content_by_lua_file...}

Answer 1

Regular expressions are very efficient at what they do.

When the task is trivial (for instance checking the presence of a particular string), a string function can be faster than a regex—depending on the platform. Here, you are checking both for a character range and a length. It's unlikely that Lua code (compiled at run time) will be faster than the pre-compiled C code of the PCRE regex library used by nginx.

In general, the regex for a string from 1 to 6 characters with "-", "_", digits and letters can be written as

^[-\w]{1,6}$

That is because

• The ^ anchor asserts that we are at the beginning of the string
• The \w word character matches letters, digits and the underscore character
• The $ anchor asserts that we are at the end of the string

However, in nginx, the ~ (request starts with) operator allows us to drop the beginning anchor ^. You would write something like this:

location ~ [-\w]{1,6}$ {
    # some rewrite code, for example
    # rewrite ^([^/]+)/?$ /oldsite/$1 break;
}

One more morsel of information for the curious: in Lua itself, the above regex could be turned into a Lua pattern, where % is used in place of \ to form metacharacters:

^[-%w]{1,6}$

Reference

• ngx_http_rewrite_module
• Lua Patterns

Answer 2

I think that in Lua you will have to check not only length, but also the content of string.
Nginx uses the C library PCRE for regular expressions.
There is also PCRE-JIT which JIT compiles regular expression, particularly useful if the regular expression is more complex than the one in your question. I think in Nginx it's faster.