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I needed a regex to find an exact number of matching digits in a string of digits. (example, "exactly three repeating-digits")

On regex101 I got a functioning regular expression (^|(.)(?!\2))(\d)\3{2}(?!\3)

but when I try it in a python3.6 shell, it fails, with match returning None:

import re
three_char_re  = re.compile("(^|(.)(?!\2))(\d)\3{2}(?!\3)")
print(three_char_re.match("14551114"))

Is there some default flag that differs between my python environment and regex101 perhaps? I cannot find a difference in the regex or the test input.

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  • So the pattern itself is correct? Commented Dec 4, 2019 at 5:58
  • 1
    @Thefourthbird i dont think so...but m not sure what OP is looking for Commented Dec 4, 2019 at 6:00
  • yes, the expression is correct. Commented Dec 4, 2019 at 7:44

3 Answers 3

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There are two problems here:

Use raw strings (see https://docs.python.org/3/reference/lexical_analysis.html) for Python regexps. Hint: ”\d” is not a backslash followed by a “d”. Change it like:

three_char_re  = re.compile(r"(^|(.)(?!\2))(\d)\3{2}(?!\3)")

Notice that it’s compile(r”...”), not compile(“...”)? The short version is that this preserves the backslashes.

Second, Use three_char_re.search or three_char_re.find_all instead of .match, so that you can find matches that aren’t at the start of the input string.

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import re
three_char_re  = re.compile(r"(^|(.)(?!\2))(\d)\3{2}(?!\3)")
print(three_char_re.findall("14551114"))

Output: [('5', '5', '1')]

You need to use findall as match starts from first character and your regex does not match from first character.

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1 Comment

also my expression didn't have the leading 'r' (eg. re.compile(r"(^|(.".
0

You need to declare your pattern as a raw string to escape \ character with prefix r.

This following pattern will search for the 3 first successive identical digits

import re
result = re.search(r"([0-9])\1\1","14551114")) 
# <re.Match object; span=(4, 7), match='111'>
print(result.group() 
# '111'
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1 Comment

yeah, the complexity is that it there must be a case of exactly 3 digits, no more no less. 1222456 True, 1222245 False, 13335660 True, 1333349990 True

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