Used to count the number of sentences in a variable. A sentence being delimited by a dot, question- or exclamation-mark (.?!).

Template:

{$articleTitle = 'Two Soviet Ships Collide - One Dies. Enraged Cow Injures Farmer with Axe.'}

{$articleTitle}
{$articleTitle|count_sentences}

output:

Two Soviet Ships Collide - One Dies. Enraged Cow Injures Farmer with Axe.
2

see also count_sentences in PHP Smarty documentation.