Regex format to find a specific string from the list

Question

This is not for homework!

Hello,

Just a quick question about Regex formatting.

I have a list of different courses.

L = ['CI101', 'CS164', 'ENGL101', 'I-', 'III-', 'MATH116', 'PSY101']

I was looking for a format to find all the words that start with I, or II, or III. Here is what I did. (I used python fyi)

for course in L:
    if re.search("(I?II?III?)*", course):
        L.pop()

I learned that ? in regex means optional. So I was thinking of making I, II, and III optional and * to include whatever follows. However, it seems like it is not working as I intended. What would be a better working format?

Thanks

re.match('^I{1,3}.*$'), please see regex101.com/r/HDS4TX/1. — Yang HG
– Yang HG, Commented Mar 20, 2019 at 3:01
@Yang Do you mind making that an answer so the question can be resolved? — Niayesh Isky
– Niayesh Isky, Commented Mar 20, 2019 at 3:57
Thanks for reminding me, I've forgot this question :) @NiayeshIsky — Yang HG
– Yang HG, Commented Mar 20, 2019 at 4:19
Yang's regex is correct, but note that your Python code won't work as intended: the pop() operation will always remove the first element in the list. Consider using a list comprehension like so: [ c for c in courses if re.match("^I{1,3}.*", c) ] — Anonymous
– Anonymous, Commented Mar 20, 2019 at 4:34

Yang HG · Accepted Answer · 2019-03-20 04:33:41Z

3

Here is the regex you should use:

^I{1,3}.*$

click here to see example

^ means the head of a line. I{1,3} means repeat I 1 to 3 times. .* means any other strings. $ means the tail of a line. So this regex will match all the words that start with I, II, or III.

Look at your regex, first, you don't have the ^ mark, so it will match I anywhere. Second, ? will only affect the previous one character, so the first I is optional, but the second I is not, then the third I is optional, the fourth and fifth I are not, the sixth I is optional. Finally, you use parentheses with *, that means the expression in parentheses will repeat many times include 0 time. So it will match 0 I, or at least 3 I.

your regex

edited Mar 20, 2019 at 4:33

answered Mar 20, 2019 at 4:30

Yang HG

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1 Comment

Isaac Over a year ago

I see. Thanks for the explanation!

Mykola Zotko · Accepted Answer · 2019-03-20 07:49:47Z

1

Instead of search() you can use the function match() that matches the pattern at the beginning of string:

import re

l = ['CI101', 'CS164', 'ENGL101', 'I-', 'III-', 'MATH116', 'PSY101']

pattern = re.compile(r'I{1,3}')

[i for i in l if not pattern.match(i)]
# ['CI101', 'CS164', 'ENGL101', 'MATH116', 'PSY101']

edited Mar 20, 2019 at 7:49

answered Mar 20, 2019 at 7:09

Mykola Zotko

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