Determining whether a floating point number is negative

ETC Puzzles
10

you can use a bit more advanced regex to handle -10.4e5 e.g. That would require (in this implementation) a look ahead not offered by bash/ksh builtins. But grep with PCRE does.

#!/bin/bash
#set -x

a='-10.43'
a='10.4e5'
a='-10.4e5'
a='5.82376'

re='^[-+]?[0-9]*[.,]?[0-9]+(?:[eE][-+]?[0-9]+)?$'

#if [[ ${a} =~ ${re} ]]; then
if [ $(grep -P "${re}" <<<"${a}") ]; then
    echo "[${a}] float"
else
    echo "[${a}] NOT float"
fi

See if it's a bit better for YOUR test cases.
You can comment out my test cases and test yours.

look here - FYI.
Here's the version based on math and bc:

#!/bin/bash
#set -x

num=-213.213332222
#num=213.213332222

if [ $(bc -l  <<< "if ( sqrt ( $num ^ 2 ) - $num == 0) 1 else 0") -eq 1 ]; then
    echo "${num} pos"
else
    echo "${num} neg"
fi