Python Regex Help Needed (Basic)

Question

I need a python regex which can help me eliminate illegal characters inside a word.

The conditions are as such:

The first character must be a-z only
All characters in the word should only be a-z (lower case) plus apostrophe ' and hyphen -
The last character must be a-z or apostrophe ' only
You can assume that the word is always lower-case

Test data:

 s = "there is' -potato 'all' around- 'the 'farm-"

Expected output:

>>>print(s)
there is' potato all' around the farm

My code is currently as such but it doesn't work correctly:

newLine = re.findall(r'[a-z][-\'a-z]*[\'a-z]?', s)

Any assistance would be greatly appreciated! Thanks!

Have you tried e.g. regex101.com/#python?

jonrsharpe
– jonrsharpe

2014-11-04 13:26:02 +00:00
Commented Nov 4, 2014 at 13:26 — jonrsharpe
– jonrsharpe, Commented Nov 4, 2014 at 13:26

Avinash Raj · Accepted Answer · 2014-11-04 13:33:36Z

1

Just match only the chars you don't want and remove ith through re.sub

>>> import re
>>> s = """potato
-potato
'human'
potatoes-"""
>>> m = re.sub(r"(?m)^['-]|-$", r'', s)
>>> print(m)
potato
potato
human'
potatoes

OR

>>> m = re.sub(r"(?m)^(['-])?([a-z'-]*?)-?$", r'\2', s)
>>> print(m)
potato
potato
human'
potatoes

DEMO

edited Nov 4, 2014 at 13:33

answered Nov 4, 2014 at 13:28

Avinash Raj

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Comments

Irshad Bhat · Accepted Answer · 2014-11-04 13:42:15Z

0

Try this:

>>> b=re.findall(r'[a-z][-\'a-z]*[\'a-z]',a)
>>> for i in b: print i
... 
potato
potato
human'
potatoes

answered Nov 4, 2014 at 13:42

Irshad Bhat

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3 Comments

python1010101 Over a year ago

I tried using your regex code but it doesn't produce the expected output you have written

Irshad Bhat Over a year ago

I have tested it on the test data you provided. It worked fine. Still, try this non-greedy version b=re.findall(r'[a-z][-\'a-z]*[\'a-z]?',a)

Harvey Over a year ago

Switch to double quotes to eliminate the escaping: b=re.findall(r"[a-z][-'a-z]*['a-z]?",a)

Vivian De Smedt · Accepted Answer · 2014-11-04 13:32:43Z

0

You can try:

[a-z][a-z'\-]*[a-z]|[a-z]

answered Nov 4, 2014 at 13:32

Vivian De Smedt

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1 Comment

python1010101 Over a year ago

thanks! it's almost accurate but I realize a case that was not caught when I ran a sample code. The beginning of the word contained apostrophe '

noob · Accepted Answer · 2014-11-04 18:01:51Z

0

Well assuming every word is separated by a space you could find all the valid words with something like this regex:

(?<= |^)[a-z](?:(?:[\-\'a-z]+)?[\'a-z])?(?= |$)

But if you want to eliminate illegal characters I guess you're better of finding the illegal characters and removing them. Now we assume again that you got a string which should only contain words which are seperated by spaces and nothing else in it.

So first of all we can sub all invalid characters out of the string: [^a-z-' ]

After doing this the only thing that could still be invalid would be a ' or - in the beginning of the word or a - in the end of the word.

So we sub those out with this regex: (?<= |^)['-]+|-+(?= |$)

edited Nov 4, 2014 at 18:01

answered Nov 4, 2014 at 17:55

noob

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