Regex to check the exact number of occurace of multiple characters - regex

Question

Wish to match a regex expression that matches with a sting when there is an exact number of occurrences of '3', '2', and '1' in a given string.
For instance, having a string "(((3x2x2+1)x2x2+1)x2+1)", I wish to have a regex expression to match exactly one occurrence of '3', five occurrences of '2', and three occurrences of '1'. If there would be more or less '3's or '2's or '1's, the regex shouldn't match.

Answer 1

I have a solution with positive and negative look aheads to do it in one regex. However, i must warn that it can become very messy very quickly. In this problem i would advise to simply count the occurrence of each number in your string without regexes.
That being said,
let's consider the following regex :
^(?=.*(?:1[^1]*){2})(?!.*(?:1[^1]*){3}).*$
^ and .*$ are there to say we want to match the whole string
?: just means we don't want to capture the group
?= is a positive lookahead, which means that our match must satisfy the condition .*(?:1[^1]*){2}
?! is a negative lookahead, which means that our match must NOT satisfy the condition .*(?:1[^1]*){3}
To summarize, you want to ALWAYS match the whole string if the positive lookahead condition is respected (digit 1 is present 2 times) and the negative lookahead condition is not (digit 1 present 3 times)
So in the example above 1*5*1 is matched, 11 is matched but 1*1*1 is not
So now, let's say you want your string to have exactly 3 '1', 1 '2', and 2 '3',
it will look like this
^(?=.*(?:1[^1]*){3})(?!.*(1[^1]*){4})(?=.*(?:2[^2]*){1})(?!.*(2[^2]*){2})(?=.*(?:3[^3]*){2})(?!.*(3[^3]*){3}).*$
It will match (1*2*1*3*1*3) but not (1*2*1*3*1*3*1) or (1*2*1*3*1)
Note that it matches (1*2*1*3*1*37) because there is a 3 in 37
Then again, i would advise against using this solution if you have too many numbers as you need to write a positive and a negative lookahead for each of your number.

Answer 2

I'd do it in 3 steps, as follows:
Mac_3.2.57$echo "(((3x2x2+1)x2x2+1)x2+1)" | egrep '^([^3]*3[^3]*){1}$' | egrep '^([^2]*2[^2]*){5}$' | egrep '^([^1]*1[^1]*){3}$'
(((3x2x2+1)x2x2+1)x2+1)
Mac_3.2.57$echo "(((3x2x2+1)x2x2+1)x2+0)" | egrep '^([^3]*3[^3]*){1}$' | egrep '^([^2]*2[^2]*){5}$' | egrep '^([^1]*1[^1]*){3}$'
Mac_3.2.57$echo "(((3x2x2+1)x2x2+1)x2+1+1)" | egrep '^([^3]*3[^3]*){1}$' | egrep '^([^2]*2[^2]*){5}$' | egrep '^([^1]*1[^1]*){3}$'
Mac_3.2.57$echo "(((3x2x2+1)x2x2+1)x0+1)" | egrep '^([^3]*3[^3]*){1}$' | egrep '^([^2]*2[^2]*){5}$' | egrep '^([^1]*1[^1]*){3}$'
Mac_3.2.57$echo "(((3x2x2+1)x2x2+1)x2+2+1)" | egrep '^([^3]*3[^3]*){1}$' | egrep '^([^2]*2[^2]*){5}$' | egrep '^([^1]*1[^1]*){3}$'
Mac_3.2.57$echo "(((0x2x2+1)x2x2+1)x2+1)" | egrep '^([^3]*3[^3]*){1}$' | egrep '^([^2]*2[^2]*){5}$' | egrep '^([^1]*1[^1]*){3}$'
Mac_3.2.57$echo "(((3X3x2x2+1)x2x2+1)x2+1)" | egrep '^([^3]*3[^3]*){1}$' | egrep '^([^2]*2[^2]*){5}$' | egrep '^([^1]*1[^1]*){3}$'
Mac_3.2.57$echo "(((2x2+1)x2x2+1)x2+1+3)" | egrep '^([^3]*3[^3]*){1}$' | egrep '^([^2]*2[^2]*){5}$' | egrep '^([^1]*1[^1]*){3}$'
(((2x2+1)x2x2+1)x2+1+3)
Mac_3.2.57$

Answer 3

You can check for each one separately and then combine the results.
\A[^3]*(3[^3]*){1}\z
\A[^2]*(2[^2]*){5}\z
\A[^1]*(1[^1]*){3}\z
Depending on your regular expressions engine, you may need to use ^ and $ instead of \A and \z.
I initially thought that it may be possible to combine them, but that would still match if there is one '3' and one '2', for example. You'll probably need code similar to this:
match = input.matches(/[123]/)
match = match && input.matches(\A[^3]*(3[^3]*){1}\z) if input.include?('3')
match = match && input.matches(\A[^2]*(2[^2]*){5}\z) if input.include?('2')
match = match && input.matches(\A[^1]*(1[^1]*){3}\z) if input.include?('1')
Note that the code above assumes that it's ok to have some but not all of these characters as long as the existing ones match the requirements.

Answer 4

It just needs a series of lookaheads to verify an exact number of specific characters.
This is a short version.
^(?=[^3]*3[^3]*$)(?=[^2]*(?:2[^2]*){5}$)(?=[^1]*(?:1[^1]*){3}$).+
https://regex101.com/r/2CJ2U6/1
^
(?= [^3]* 3 [^3]* $ ) # 1 of three
(?= # 5 of two
[^2]*
(?: 2 [^2]* ){5}
$
)
(?= # 3 of one
[^1]*
(?: 1 [^1]* ){3}
$
)
.+