poh-tay-toh poh-tah-toh, does writing the same code a different way affect readability?

I have a particular if statement that could be written in different ways, and I'm curious as to whether there's any significant difference in readability that I should prefer one over the other:

The flow can be boiled down to

if string does not begin with '_'
    do stuff
else
    throw error

Some different ways of writing the if condition:

str[0] !== '_'
str.indexOf('_')
!/^_/.test(str)
/^[^_]/.test(str)

I think the first is the simplest and the most readable one from the list but I'd prefer the charAt function:

x.charAt(0) !== '_'

Furthermore, I'd reverse the condition:

if string begins with '_' {
    throw error
}
do stuff

Reference:

• How to get first character of string?

• Flattening Arrow Code

StackExchange Code Review Q#12352, answer score: 12