Such sizes are quite valid but were never tested.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Shows on my laptop (6 iterations processed by benchstat):

name         time/op
ParseSize-4  10.6µs ± 3%

name         alloc/op
ParseSize-4  3.26kB ± 0%

name         allocs/op
ParseSize-4    72.0 ± 0%

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

This way, error messages will show correct line information.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Co-authored-by: Kir Kolyshkin <kolyshkin@gmail.com>

Using a regular expression brings in the whole regexp and regexp/syntax
packages, which increase the resulting binary size by about 130K
(Go 1.18.1, Linux/amd64).

Besides, regular expressions are generally slow and incur some
initialization overhead (to compile all global regexp.MustComplile
variables). This, unlike the size difference, is not the main motivation
for this commit, but it feels like it should have also been mentioned.

A quick benchmark comparison shows a huge improvement (again, this is
not why this is done, nevertheless it pleases the eye):

name         old time/op    new time/op    delta
ParseSize-4    10.6µs ± 3%     2.6µs ±29%  -75.10%  (p=0.002 n=6+6)

name         old alloc/op   new alloc/op   delta
ParseSize-4    3.26kB ± 0%    0.20kB ± 0%  -93.75%  (p=0.000 n=7+6)

name         old allocs/op  new allocs/op  delta
ParseSize-4      72.0 ± 0%      26.0 ± 0%  -63.89%  (p=0.000 n=7+6)

Compatibility note: As a result, we are now a but more relaxed to the
input, allowing e.g. ".4 Gb", or "-0", or "234. B", following the rules
of strconv.ParseFloat. It seems that those were previously rejected as
a result of a regex being used, not deliberately.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Remove regexp use