Confused about GNU `sort(1)` of a numerical sub field

Question

I wish to sort a space separated table, with the numerical value that found on the 2nd field. I can assume that the 2nd field is always fooN but the length of N is unknown:

antiq. foo11 girls
colleaguing foo2 Leinsdorf
Cousy foo0 Montgomeryville
bowlegged foo1 pollack
Chevrier foo10 ill-conceived
candlebomb foo3 seventieths
autochthony foo101 re-enable
beneficiate foo100 osteometric

I read man sort(1) and played with all sort of options. On my system I found the line:

sort -n -k2.5 table

to work.

My question is why?

According to the man page:

-k, --key=POS1[,POS2]
   start a key at POS1, end it at POS 2 (origin 1) 
...
POS is F[.C][OPTS], where F is the field number and C the characterposition in the
field. OPTS is one or more single-letter ordering options, which override global
ordering options for that key. If no key is given, use the entire line as the key. 

So why sort -n -k2.4 table don't work and sort -n -k2.5 does?

Answer 1

The answer is: the leading space(s) are counted as part of the field, unless:

sort -b -n -k2.4 table

or curiously:

LC_ALL=C sort -t" " -n -k2.4 table

that also yields the correct result.

---

... and one more thing ...

It seem that it is better to use:

sort -b -n -k2.4,2 table

and thus limit the sort to the end of the 2nd field.

Answer 2

I don't know if it helps but info sort states the following:

sort -t : -k 2,2n -k 5.3,5.4

 Note that if you had written `-k 2' instead of `-k 2,2' `sort'
 would have used all characters beginning in the second field and
 extending to the end of the line as the primary _numeric_ key.
 For the large majority of applications, treating keys spanning
 more than one field as numeric will not do what you expect.

Perhaps try adding a space between -k and 2, or try setting POS2?