backticks - PS1 command substitution fails when containing newlines on msys bash

Question

This command succeeds

$ PS1='$(date +%s) $ '
1391380852 $

However if I add a newline it fails

$ PS1='$(date +%s)
$ '
bash: command substitution: line 1: syntax error near unexpected token `)'
bash: command substitution: line 1: `date +%s)'

If I use backticks it works

$ PS1='`date +%s`
$ '
1391381008
$

but backticks are discouraged. So what is causing this error?

GNU bash, version 4.2.45(6)-release

Answer 1

You can disambiguate the parsing easily, to prevent hitting any such bug (though I can't reproduce it myself):

PS1='$(date +%s)'$'
$ '

This $' ' syntax parses to a literal newline character, whereas ' ' parses to a string containing a two-character escape sequence.

For more info on how $'' differs from '' (expanding backslash-escaped sequences) refer to the Bash Hackers Wiki.