Use whitespaceBoundary to divide a string along whitespace boundaries. This pattern matches boundaries between a whitespace character and a non-whitespace character.

Create a pattern that matches any whitespace boundaries.

txt = "123 abc .?! def 456";
pat = whitespaceBoundary ;

Use replace to insert "|" characters at the matched boundaries.

replace(txt,pat,"|")

ans = 
"123| |abc| |.?!| |def| |456"

Use the "start" and "end" options for whitespaceBoundary to match the boundary between whitespaces and non-whitespace characters.

Create a string that contains several different character types. Create a pattern that matches any characters between an "end" boundary of whitespaces and a "start" boundary.

txt = "123 abc .?! def 456";
pat = whitespaceBoundary("end") + wildcardPattern(1,inf) + whitespaceBoundary("start");

Extract the pattern.

boundaries = extract(txt,pat)

boundaries = 3x1 string
    "abc"
    ".?!"
    "def"

Use the ~ operator to negate whitespaceBoundary. This matches boundaries between two characters when both are whitespace characters or neither is a whitespace character.

Create a string that contains several different character types. Create a pattern that matches a negated whitespaceBoundary.

txt = "123 abc .?!";
pat = ~whitespaceBoundary;

Use replace to insert "|" characters to show where ~whitespaceBoundary matches.

boundaries = replace(txt,pat,"|")

boundaries = 
"|1|2|3 a|b|c .|?|!|"