Just for fun:

This seems to work if the input is surrounded by exactly one whitespace, but don't try to escape doublequotes

• $1 is the whitespace
• $2 the optional doublequote
• $3 the enclosed text

  DB<89> p "'$_'"
' 23    45.67 "John Marcus" Surname 23 45.67 "John Marcus" Surname ext
+ra '
  DB<90> say "'$3'" while /\s*(\s)("?)(.*?)\2(?=\1)/g
'23'
'45.67'
'John Marcus'
'Surname'
'23'
'45.67'
'John Marcus'
'Surname'
'extra'

  DB<91>
[download]

For testing I'd suggest to automatically create strings for random input. Like this you can cover a large set of cases.

NB: here are still dragons.