Python 2, 26 bytes

from textwrap import*

fill

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Meh... built-ins are boring... instead, have a nice 87-byte solution here:

s,n=input()

x=''

for i in s.split():c=n<len(x+i);exec'print x'*c;x=x*-~-c+i+' '

print x

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Outputs trailing spaces.

Perl 6, 46 29 bytes

{;*.comb(/.**{1..$_}[s|$]/)}

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Regex based solution that takes input curried, like f(n)(s) and returns a list of lines. Each line except the last has a trailing whitespace

Explanation:

{;*                         }   # Anonymous code block that returns a Whatever lambda

   .comb(/                /)    # Split the string by

          .**{1..$_}            # Up to n characters

                    [s|$]      # Terminated by a whitespace char or the end of the string

PHP, 8 bytes

Admittedly not the most original solution, but PHP has a native function that matches your requirements perfectly!

wordwrap:

string wordwrap ( string $str [, int $width = 75 [, string $break = "n" [, bool $cut = FALSE ]]] )

Wraps a string to a given number of characters using a string break character.

Use like so:

$str = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed eget erat lectus. Morbi mi mi, fringilla sed suscipit ullamcorper, tristique at mauris. Morbi non commodo nibh. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed at iaculis mauris. Praesent a sem augue. Nulla lectus sapien, auctor nec pharetra eu, tincidunt ac diam. Sed ligula arcu, aliquam quis velit aliquam, dictum varius erat.";

echo wordwrap($str, 50);

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JavaScript (ES6),  75 73  72 bytes

Takes input as (string)(n).

s=>n=>s.split` `.map(w=>r=(u=r?r+' '+w:w)[n]?(o+=r+`

`,w):u,o=r='')&&o+r

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Variables

The formatted output is stored in $o$ (in green below).

The updated line $u$ is defined as the concatenation of:

• the current line $r$ (in black below)


• a space if $r$ is not empty, or nothing otherwise (in orange below)


• the new word $w$ (in blue below)

We need to insert a line break whenever the $n$-th character of $u$ is set (0-indexed, in red below).

Example

$n=16$ and $s$ = "LOREM IPSUM DOLOR"

Adding "LOREM":
$$smallbegin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}
hline
00&01&02&03&04&05&06&07&08&09&10&11&12&13&14&15&color{red}{16}\ hline
color{blue}L&color{blue}O&color{blue}R&color{blue}E&color{blue}M&&&&&&&&&&&&\ hlineend{array}
$$

Adding "IPSUM":
$$smallbegin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}
hline
00&01&02&03&04&05&06&07&08&09&10&11&12&13&14&15&color{red}{16}\ hline
L&O&R&E&M&color{orange}bullet&color{blue}I&color{blue}P&color{blue}S&color{blue}U&color{blue}M&&&&&&\ hlineend{array}
$$

Adding "DOLOR":
$$smallbegin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}
hline
00&01&02&03&04&05&06&07&08&09&10&11&12&13&14&15&color{red}{16}\ hline
L&O&R&E&M&bullet&I&P&S&U&M&color{orange}bullet&color{blue}D&color{blue}O&color{blue}L&color{blue}O&color{blue}R\ hlineend{array}
$$

$$smallbegin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}
hline
00&01&02&03&04&05&06&07&08&09&10&11&12&13&14&15&color{red}{16}\ hline
color{green}L&color{green}O&color{green}R&color{green}E&color{green}M&color{green}bullet&color{green}I&color{green}P&color{green}S&color{green}U&color{green}M&color{green}hookleftarrow&&&&&\ hline
D&O&L&O&R&&&&&&&&&&&&\ hlineend{array}
$$

Vim, 15 bytes/keystrokes

DJ:se tw=<C-r>"

gq_

A text formatting question? I know just the tool for the job! And it even has my name in the first two keystrokes :D

<C-r> means ctrl-r.

This could ever so slightly shorter in V, but I prefer answering in vanilla vim for answers that really show off how concise vim can be for the right challenge. And the difference is so small anyway.

This could also be the following for 15 bytes as well:

:se tw=<C-r><C-w>

ddgq_

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Haskell, 70 bytes

s!n|length s<=n=[s]|(t,_:d)<-splitAt(until((<'!').(s!!))pred n)s=t:d!n

Python 2, 74 bytes

s,n=input()

while s:i=n;exec"i-=' '<(s+' '*n)[i];"*n;print s[:i];s=s[i+1:]

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R, 36 27 bytes

R has this as a built-in (strwrap), we return a vector of split lines.

function(s,n)strwrap(s,n+1)

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Retina 0.8.2, 37 bytes

.+$

$*

!`(?=S.*¶(1)+)(?<-1>.)+(?=s)

Try it online! Takes s and n on separate lines. Explanation:

.+$

$*

Convert n to unary.

(?=S.*¶(1)+)(?<-1>.)+(?=s)

Match non-whitespace, then look ahead to n and count it as $#1. Then go back and use a balancing group to match up to n characters followed by whitespace.

!`

Output the matches as a list of lines.

Charcoal, 19 bytes

Ｎθ←Ｆ⪪Ｓ «¿‹⁺ＬιⅈθＭ→⸿ι

Try it online! Link is to verbose version of code. Takes input of n and s on separate lines. Explanation:

Ｎθ

Input n.

←

Move the cursor left one square to balance the right movement from the first iteration of the loop.

Ｆ⪪Ｓ «

Split the string on spaces and loop over the words.

¿‹⁺Ｌιⅈθ

Calculate whether the next word will reach the right edge.

Ｍ→

If it will not then move one square right.

⸿

If it will then start a new line.

ι

Output the word.

Japt, 25 bytes

¸rÈ·Ì+S+Y Ê>V?X+R+Y:X+S+Y

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Pretty straightforward. I'm certain there's a more concise way to do this than having "X+R/S+Y" three times, but my alternatives have all been longer.

Explanation:

¸                            :Split into words

 r                           :Reduce back into a string by:

  È                          : Start with the already reduced portion

   ·Ì                        : Get the last line

     +S+Y                    : Add a space and the next word

          Ê>V?               : If the length of that line is greater than n:

              X+R+Y          :  Use a newline before the next word

                   :         : Otherwise:

                    X+S+Y    :  Use a space before the next word

Red, 125, 117, 114 112 bytes

func[s n][d: 0 parse s[any[to" "p:" "opt[[to" "| to end]q:(if(-1 - d + index? q)> n[p/1: #"^/"d: index? p])]]]s]

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JavaScript, 39 bytes

s=>n=>eval(`s.match(/.{1,${n}} |.+/g)`)

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Bash/Coreutils, 8 bytes

fold -sw <n>

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The fold utility, that's bundled along with coreutils, does exactly what the question asks. The -s flag makes it so that it only breaks on whitespaces(ie. when the word ends), the value of n is passed using the argument -w. Input is accepted via stdin

Thanks to @Erik the Outgolfer, a golfed version :

Python 3, 94 bytes

def f(t,n):

 while t:i=n+(t[min(len(t)-1,n)]==" "or-t[n-1::-1].find(' '));print(t[:i]);t=t[i:]

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# Python 3, 130 bytes

def f(t,n):

 l=

 while len(t):

  i=(n-t[:n][::-1].find(' '),n+1)[t[min(len(t)-1,n)]==" "]

  l.append(t[:i])

  t=t[i::]

 return l

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Not so golfed version...

JavaScript + HTML + CSS, 117 64 bytes

-53 bytes courtesy of @Neil

n=50

s="Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed eget erat lectus. Morbi mi mi, fringilla sed suscipit ullamcorper, tristique at mauris. Morbi non commodo nibh. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed at iaculis mauris. Praesent a sem augue. Nulla lectus sapien, auctor nec pharetra eu, tincidunt ac diam. Sed ligula arcu, aliquam quis velit aliquam, dictum varius erat."

f=(n,s)=>document.body.innerHTML+=`<tt><p style=width:${n}ch>${s}`

f(n,s)

C# (Visual C# Interactive Compiler), 91 bytes

s=>n=>s.Split(' ').Aggregate((a,w)=>a+(a.Length-a.LastIndexOf('n')+w.Length>n?'n':' ')+w)

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Powershell, 40 bytes

param($s,$n)$s-replace"(.{0,$n}) ",'$1

'

Note: The regexp uses a space as a separator because the author posts the yes on the question Are spaces the only characters that divide words?. The reqexp should be (.{0,$n})s if any whitespace allowed.

Test script:

$f = {



param($s,$n)$s-replace"(.{0,$n}) ",'$1

'



}



@(

,(50, "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed eget erat lectus. Morbi mi mi, fringilla sed suscipit ullamcorper, tristique at mauris. Morbi non commodo nibh. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed at iaculis mauris. Praesent a sem augue. Nulla lectus sapien, auctor nec pharetra eu, tincidunt ac diam. Sed ligula arcu, aliquam quis velit aliquam, dictum varius erat.",

@"

Lorem ipsum dolor sit amet, consectetur adipiscing

elit. Sed eget erat lectus. Morbi mi mi, fringilla

sed suscipit ullamcorper, tristique at mauris.

Morbi non commodo nibh. Pellentesque habitant

morbi tristique senectus et netus et malesuada

fames ac turpis egestas. Sed at iaculis mauris.

Praesent a sem augue. Nulla lectus sapien, auctor

nec pharetra eu, tincidunt ac diam. Sed ligula

arcu, aliquam quis velit aliquam, dictum varius

erat.

"@)

) | %{

    $n,$s,$expected = $_

    $result = &$f $s $n

    $result-eq$expected

    # $result   # uncomment this line to dispaly a result

}

Output:

True

Mathematica, 16 bytes

InsertLinebreaks

Built-in function. Takes a string and an integer as input and returns a string as output.

InsertLinebreaks["string", n]

 inserts newline characters to make no line longer than n characters.

Jelly, 12 bytes

ḲŒṖK€€ḣ€ƑƇṪY

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Unfortunately, this is too slow to work for the provided test case in under a minute over TIO.

C# (.NET Core), 162 bytes

stringt(string n,int a){var g="";for(int i=0;i++<Math.Floor((double)n.Length/a);)g+=$"^.{{{i*a-1}}}|";return Regex.Split(n,$@"(?n)(?<=({g.Trim('|')})S*)s");}}

This function uses a regex which matches the closest whitespace that is near the nth or multiple of nth character and splits the string based on it.

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The TIO link is a full program, and the function has a static keyword so the function can be called from main.

Test Regex