Regex to match a string based on particular character count

Question

It is not a duplicate question, other questions are about the repetition of a regex and my question is how can we get/limit a particular character count in regex for validation, I am looking for a regex to match a string only when the string has the count of character ' as 1. Example:

patt = #IDontKnow
s = "Shubham's"
if re.match(patt, s):
    print ("The string has only ONE '")
else:
    print ("The String has either less or more than ONE ' count")

What have you tried? Please provide [MCVE](https://stackoverflow.com/help/mcve). — sophros, Nov 20 '18 at 10:00
@MCVE, I am very new to regex and could not get any pattern yet. — Shubham Shrivastava, Nov 21 '18 at 05:45
Why won't you have a read first of [a manual](https://www.pythoncentral.io/introduction-to-python-regular-expressions/) and if you arrive at any issues then post your attempt? — sophros, Nov 21 '18 at 07:27
@sophros, I have tried what I could do, but I am not getting any way in regex to get the character count and compare at the same time. — Shubham Shrivastava, Nov 22 '18 at 05:49

Mark · Answer 1 · 2018-11-20T10:03:44.027

0

Why not just use .count()?

s = "Shubham's"
if s.count("\'") == 1:
    print ("The string has only ONE '")
else:
    print ("The String has either less or more than ONE ' count")

edited Nov 20 '18 at 10:03

answered Nov 20 '18 at 10:01

Mark

5,089
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Eggsactly what I was thinking. Python is much more than a wrapper for unnecessary complicated regexes. – Jongware Nov 20 '18 at 10:02
@Mark, I need a regex, I have to give it as a validator in wtforms. – Shubham Shrivastava Nov 21 '18 at 05:44

score 0 · Accepted Answer · answered Nov 22 '18 at 06:38

I guess what you are looking for is this:

import re
pat = "^[^\']*\'[^\']*$"
print (re.match(pat, "aeh'3q4'bl;5hkj5l;ebj3'"))
print (re.match(pat, "aeh3q4'bl;5hkj5l;ebj3'"))
print (re.match(pat, "aeh3q4bl;5hkj5l;ebj3'"))
print (re.match(pat, "'"))
print (re.match(pat, ""))

Which gives output:

None
None
<_sre.SRE_Match object; span=(0, 21), match="aeh3q4bl;5hkj5l;ebj3'">
<_sre.SRE_Match object; span=(0, 1), match="'">
None

What "^[^\']*\'[^\']*$" does?

^ matches the beginning of string

[^\']* - * matches 0 or more characters from set defined in []. Here, we have a set negated using ^. The set is defined as one character - ', which is escaped so it looks \'. Altogether, this group matches any number of any characters except '

\' - matches one and only character '

$ - matches end of the string. Without it partial matches would be possible which could contain more ' characters. You can compare with above:

print (re.match("^[^\']*\'[^\']*", "aeh'3q4'bl;5hkj5l;ebj3'"))

<_sre.SRE_Match object; span=(0, 7), match="aeh'3q4">

Exaaaactly!! Thank you so much. But unfortunately, I have got many downvotes for it. :( — Shubham Shrivastava, Nov 23 '18 at 06:38
This is also useful in text editors to find missing closing quotes in text templates (pug, jade,...). Thank you. — donmartin, Nov 25 '19 at 10:05

Regex to match a string based on particular character count

2 Answers2