Read ieee-754-2008 four-byte floating-point number from binary file?

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I'm reading some files in an existing binary format, which include 4-byte floating-point numbers. I have the file's contents loaded into a string. I need a function which will parse one floating-point number from the string, given the index of the first byte. Does Lua's library provide this? Or should I throw something together with the bitop library? 

This is using Lua 5.1.4. 

Thanks all,
-Max.