Something like: 
$s =~ s/(?<=(?<!\d(?=[.,]\d))\p{P})(?!\s)/ /g;

[download]
Taking that apart, we are looking for a spot between two characters where a space should be inserted. We have a positive requirement for what comes before the spot (that's the (?<=...)) and a negative requirement for what comes after the spot. The after condition is simple. If there is a whitespace character (I'm guessing that you mean to include tab, newline, etc., based on your use of \S) this spot can be skipped.

The before condition is essentially (?<=\pP) with an additional condition. \pP matches any punctuation character. These are: 

$ perl -wle'use charnames (); for (0..255) { chr($_) =~ /\pP/ and prin
+t chr($_)
," ($_): ", charnames::viacode($_) }'
! (33): EXCLAMATION MARK
" (34): QUOTATION MARK
# (35): NUMBER SIGN
% (37): PERCENT SIGN
& (38): AMPERSAND
' (39): APOSTROPHE
( (40): LEFT PARENTHESIS
) (41): RIGHT PARENTHESIS
* (42): ASTERISK
, (44): COMMA
- (45): HYPHEN-MINUS
. (46): FULL STOP
/ (47): SOLIDUS
: (58): COLON
; (59): SEMICOLON
? (63): QUESTION MARK
@ (64): COMMERCIAL AT
[ (91): LEFT SQUARE BRACKET
\ (92): REVERSE SOLIDUS
] (93): RIGHT SQUARE BRACKET
_ (95): LOW LINE
{ (123): LEFT CURLY BRACKET
} (125): RIGHT CURLY BRACKET
¡ (161): INVERTED EXCLAMATION MARK
« (171): LEFT-POINTING DOUBLE ANGLE QUOTATION MARK
· (183): MIDDLE DOT
» (187): RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
¿ (191): INVERTED QUESTION MARK

[download]
Plus several more unicode characters if you have utf8 data. If you mean a more restricted set of characters, use a character class like [.!,;] or whatever.

The additional condition is (?<!\d(?=[.,]\d)): this stipulates that what comes before the punctuation should not match \d(?=[.,]\d); that is a single digit, where the digit is followed by a period or comma and another digit. Note that that lookahead is actually escaping the bounds of the outer lookbehind, but that is perfectly ok to do.

Update: added actual character to punctuation table

Update: weird. backtick (GRAVE ACCENT) isn't considered punctuation

In reply to Re: Regex to add space after punctuation sign by ysth
in thread Regex to add space after punctuation sign by dda

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