Python - First and last character in string must be alpha numeric, else delete

Question

I am wondering how I can implement a string check, where I want to make sure that the first (&last) character of the string is alphanumeric. I am aware of the isalnum, but how do I use this to implement this check/substitution?

So, I have a string like so:

st="-jkkujkl-ghjkjhkj*"

and I would want back:

st="jkkujkl-ghjkjhkj"

Thanks..

score 5 · Accepted Answer · answered Oct 16 '16 at 07:40

5

Though not exactly what you want, but using str.strip should serve your purpose

import string

st.strip(string.punctuation)
Out[174]: 'jkkujkl-ghjkjhkj'

answered Oct 16 '16 at 07:40

Abhijit

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@Abhijit: Awesome! Thanks for the VERY pythonic and regex(less) answer! – AJW Oct 16 '16 at 08:21

sisanared · Answer 2 · 2016-10-16T08:13:53.167

2

You could use regex like shown below:

import re
# \W is a set of all special chars, and also include '_'
# If you have elements in the set [\W_] at start and end, replace with ''
p = re.compile(r'^[\W_]+|[\W_]+$') 
st="-jkkujkl-ghjkjhkj*"
print p.subn('', st)[0]

Output: jkkujkl-ghjkjhkj

Edit:

If your special chars are in the set: !"#$%&\'()*+,-./:;<=>?@[\]^_`{|}~

@Abhijit's answer is much simpler and cleaner.

If you are not sure then this regex version is better.

edited Oct 16 '16 at 08:13

answered Oct 16 '16 at 07:38

sisanared

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I used something similar, but You can add + or * to re expression, in order to remove multiple occurrences of non-alpha chars. – Fejs Oct 16 '16 at 07:48
Sounds like a good idea. Thank you I'll edit the regex. – sisanared Oct 16 '16 at 07:49

score 1 · Answer 3 · answered Oct 16 '16 at 07:45

1

You can use following two expressions:

st = re.sub('^\W*', '', st)
st = re.sub('\W*$', '', st)

This will strip all non alpha chars of the beginning and the end of the string, not just the first ones.

answered Oct 16 '16 at 07:45

Fejs

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better to use `+` though – Antti Haapala -- Слава Україні Oct 16 '16 at 07:50
Can You explain why? – Fejs Oct 16 '16 at 07:58
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@Fejs ^\W* will match a string 'foo', so its a wasted cycle of substitution. You can try doing `st = re.sub('^\W*', '-', st)` on a string of value `foo` and you will see a `-` at the front – sisanared Oct 16 '16 at 08:15
Yep, just do `re.sub('\\*', '+')` *to your answer* and it is perfect ;) – Antti Haapala -- Слава Україні Oct 16 '16 at 08:29

score 0 · Answer 4 · answered Oct 16 '16 at 07:42

0

You could use a regular expression.

Something like this could work;

\w.+?\w

However I'm don't know how to do a regexp match in python..

answered Oct 16 '16 at 07:42

Koen.

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score 0 · Answer 5 · answered Oct 16 '16 at 07:43

0

hint 1: ord() can covert a letter to a character number

hint 2: alpha charterers are between 97 and 122 in ord()

hint 3: st[0] will return the first letter in string st[-1] will return the last

answered Oct 16 '16 at 07:43

Whud

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Christodoulos Fragkoudakis · Answer 6 · 2016-10-16T09:02:55.133

0

An exact answer to your question may be the following:

def stringCheck(astring):
    firstChar = astring[0] if astring[0].isalnum() else ''
    lastChar = astring[-1] if astring[-1].isalnum() else ''
    return firstChar + astring[1:-1] + lastChar

edited Oct 16 '16 at 09:02

answered Oct 16 '16 at 08:56

Christodoulos Fragkoudakis

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Python - First and last character in string must be alpha numeric, else delete

6 Answers6