[Tutor] regexp

[Steven D'Aprano]

Dinara Vakhitova wrote:
> Hello,
> 
> I need to find the words in a corpus, which letters are in the alphabetical
> order ("almost", "my" etc.)

Quoting Jamie Zawinski:

     Some people, when confronted with a problem, think "I know, I'll
     use regular expressions." Now they have two problems.

Now you have two problems: find words in the corpus which are in 
alphabetical order, and get the damn regular expression to work correctly.

Don't use a regex for this. It is much simpler to write a Python 
function to solve it:

def letters_in_order(astring):
     """Return True if letters in astring are in alphabetical order.

     >>> letters_in_order("almost")
     True
     >>> letters_in_order("zoology")
     False

     """
     if len(astring) <= 1:
         return True
     for i in range(1, len(astring)):
         if astring[i] < astring[i-1]:
            # Pair of characters are out of order.
             return False
     # If none of the pairs are out of order, they whole string
     # must be in order.
     return True

words = filter(letters_in_order, corpus)
for word in words:
     print(word)



-- 
Steven