Detecting loops in in in strings

Perl, 36 bytes

#!perl -p
s!(.+)\1+!@{[$&=~/\Q$1/g]}."[$1]"!eg

Try it online!

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PHP 5.6, 68 bytes

Same code in PHP. Requires the command line option -F, and relies on deprecated /e modifier.

<?=preg_filter('/(.+)\1+/e','substr_count("\0","\1")."[\1]"',$argn);

C, 224 205 203 199 195 193 chars

The string is given as program parameter. As usual, use double quotes if your string contains spaces, and for empty strings - calling the program without any parameter crashes, but calling with "" works.

Output for test cases is:

st st st Hello ThereThere => 3[st ]He2[l]o 2[There]
Bookkeeper                => B2[o]2[k]2[e]per
/\/\/\\\\\\\\/\/\/        => 3[/\]2[\\\]3[\/]

which looks correct to me regarding "choose the largest possible repeating section".

Code:

l,r,f,i;char*c;main(a,b)char**b;{l=strlen(c=b[1]);for(;putchar(*c);c++)for(a=l;--a;f=0)for(r=a;!f;r+=a){for(i=0;i<a;)f|=r>l||c[i]-c[r+i++];if(f*r>a)c[a]=0,printf("\b%i[%s]",r/a,c),c+=r-1,a=1;}}

scala, 102 chars

print("(?!\\s)(.+)\\1+".r.replaceAllIn(readLine,m=>m.matched.size/m.group(1).size+"["+m.group(1)+"]"))

Output

st st st Hello ThereThere ->    3[st ]He2[l]o 2[There]
st Hello st st Bye        ->    st He2[l]o 2[st ]Bye
Bookkeeper                ->    B2[o]2[k]2[e]per
/\/\/\\\\\\\\/\/\/        ->    3[/\]2[\\\]3[\/]