Trim string (both left and right) to nearest word or sentence

I'm writing a function that finds a string near a identical string(s) in a larger piece of text. So far so good, just not pretty.

I'm having trouble trimming the resulting string to the nearest sentence/whole word, without leaving any characters hanging over. The trim distance is based on a number of words either side of the keyword.

keyword = "marble"
string = "Right. This marble is as slippery as this marble. Kwoo-oooo-waaa! Ahhhk!"

with 1 word distance (either side of key word) it should result in:
2 occurrences found
"This marble is..."
"...this marble. Kwoo-oooo-waaa!"

with 2 word distance:
2 occurrences found
"Right. This marble is as..."
"...as this marble. Kwoo-oooo-waaa! Ahhhk!"

what I've got so far is based on character, not word distance.

2 occurrences found
"ght. This marble is as sli"
"y as this marble. Kwoo-ooo"

However a regex could split it to the nearest whole word or sentence. Is that the most Pythonic way to achieve this? This is what I've got so far:

import re

def trim_string(s, num):
  trimmed = re.sub(r"^(.{num}[^\s]*).*", "$1", s) # will only trim from left and not very well
  #^(.*)(marble)(.+) # only finds second occurrence???

  return trimmed

s = "Right. This marble is as slippery as this marble. Kwoo-oooo-waaa! Ahhhk!"
t = "Marble"


if t.lower() in s.lower():

  count = s.lower().count(t.lower())
  print ("%s occurrences of %s" %(count, t))

  original_s = s

  for i in range (0, count):
    idx = s.index(t.lower())
    # print idx

    dist = 10
    start = idx-dist
    end = len(t) + idx+dist
    a = s[start:end]

    print a
    print trim_string(a,5)

    s = s[idx+len(t):]

Thank you.