Is it possible to trim it from the beginning with % formatting?

Python's % formatting comes from C's printf.

Note that the . indicates precision for a float. That it works on a string is a mere side effect, and unfortunately, there is no provision in the string formatting specification to accommodate stripping a string from the left to a fixed max width.

Therefore if you must strip a string to a fixed width from the end, I recommend to slice from a negative index. This operation is robust, and won't fail if the string is less than 10 chars.

>>> up_to_last_10_slice = slice(-10, None)
>>> 'Lorem Ipsum'[up_to_last_10_slice]
'orem Ipsum'
>>> 'Ipsum'[up_to_last_10_slice]
'Ipsum'

str.format also no help

str.format is of no help here, the width is a minimum width:

>>> '{lorem:>10}'.format(lorem='Lorem Ipsum')
'Lorem Ipsum'
>>> '{lorem:*>10}'.format(lorem='Lorem')
'*****Lorem'

(The asterisk, "*", is the fill character.)

Question 1: Restrict to certain characters

You are right, this is easy to solve with regular expressions:

import re

input_str = raw_input("Please provide some info: ")
if not re.match("^[a-z]*$", input_str):
    print "Error! Only letters a-z allowed!"
    sys.exit()

Question 2: Restrict to certain length

As Tim mentioned correctly, you can do this by adapting the regular expression in the first example to only allow a certain number of letters. You can also manually check the length like this:

input_str = raw_input("Please provide some info: ")
if len(input_str) > 15:
    print "Error! Only 15 characters allowed!"
    sys.exit()

Or both in one:

import re

input_str = raw_input("Please provide some info: ")
if not re.match("^[a-z]*$", input_str):
    print "Error! Only letters a-z allowed!"
    sys.exit()
elif len(input_str) > 15:
    print "Error! Only 15 characters allowed!"
    sys.exit()

print "Your input was:", input_str

info = (data[:75] + '..') if len(data) > 75 else data

This code matches the JavaScript, but you should consider using data[:73] so that the total result including the .. fits in 75 characters.

Python's input function cannot do this directly; but you can truncate the returned string, or repeat until the result is short enough.

# method 1
answer = input("What's up, doc? ")[:10]  # no more than 10 characters

# method 2
while True:
    answer = input("What's up, doc? ")
    if len(answer) <= 10:
        break
    else:
        print("Too much info - keep it shorter!")

If that's not what you're asking, you need to make your question more specific.