Several functions including LOOKUP, VLOOKUP do not work correctly if the setting 
"Search criteria = and <> must apply to whole cells" is not "ON"
There is no reference to this in the help files.

I would expect this to apply to
  HLOOKUP, LOOKUP, MATCH, VLOOKUP  

Each help reference for such functions needs a specific reference to this.

I suggest the following text should be added after the statement on regular expressions.
"To operate correctly with strings, [function-name] requires that the condition 'Search criteria = and <> must apply to whole cells' is ON.  You can switch this on and off in Tools - Options - OpenOffice Calc - Calculate."

In addition, there should be a reverse statement under the help title
"Search criteria = and <> must apply to whole cells"
This criterion should be on if the following functions are used: HLOOKUP,LOOKUP, MATCH, VLOOKUP. The effect of the 

The problem is severe enough to need hours of time to solve if there is nothing in 'help' files.

I would expect this to apply to
  HLOOKUP, LOOKUP, MATCH, VLOOKUP  

Issue 126362 applies. I can find simpler test examples if required, but will need time to set up.