# Regular expression: how to search for a sequence of one number alternated with 0s?

조회 수: 2(최근 30일)
Serbring 2021년 8월 11일
댓글: Serbring 2021년 8월 17일
Hi all,
I need to craft a regular expression. What I need is to search any pattern where sequences of one number (i.e., 1 or 2) is alternated with a sequence of 0s. So, I search for patterns like the following:
'10111'
'22200002'
'300333'
How can I do it?
Thank you.
Best regards.
MM
##### 댓글 수: 1표시숨기기 없음
Walter Roberson 2021년 8월 12일
편집: Walter Roberson 2021년 8월 12일
Does the sequence always end with the same number? For example would 300330000 be valid? Would 300334 be valid on the grounds that it starts with alternating sequence, and then anything can be after that?
Is "no" alternations also valid? Such as 222 by itself, which is "a sequence of twos, followed by zero repetitions of (0's and 2's) ?
Is the pattern only ever at the beginning of the string, or could it be inside the string as well? If it is inside then that answers the first two questions.

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### 채택된 답변

DGM 2021년 8월 12일
Well, I'm certainly not the one to ask about regex. I thought I had this nailed down, but had to resort to this to get it to work in MATLAB. I bet it could be simpler.
C = regexp(vec,'(\d)\1*0+\1+','match')
C = 1×6 cell array
{'55055'} {'3333003'} {'440444'} {'500055'} {'404'} {'44000444'}
(\d) matches and captures any numeric digit
\1* matches zero or more instances of the captured digit
0+ matches one or more zeros
\1+ matches one or more instances of the captured digit
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Serbring 2021년 8월 17일
Thank you so much. Is it possible to introduce tokens in lookahead/lookbehind expressions? I tried with the following, but it does not work. I have read in the web, that certains regular expression engines are not able to deal with tokens in was not able to do it.
C = regexp(S,'(?<=(\d)\1*)0+(?=(\1)+)','match')

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### 추가 답변(1개)

Stephen23 2021년 8월 12일
편집: Stephen23 2021년 8월 12일
C = regexp(S,'(\d)\1*(0+\1+)+','match')
C = 1×7 cell array
{'55055'} {'0000'} {'3333003'} {'4404440044'} {'500055'} {'404'} {'440040444'}
Note that \d also matches the zero character. If your definition of "one number" excludes zero, then use this instead:
C = regexp(S,'([1-9])\1*(0+\1+)+','match')
C = 1×6 cell array
{'55055'} {'3333003'} {'4404440044'} {'500055'} {'404'} {'440040444'}
##### 댓글 수: 1표시숨기기 없음
DGM 2021년 8월 12일
편집: DGM 2021년 8월 12일
See, excluding zero is the kind of obvious thing I expected I'd miss.
I didn't even think about continuing the alternation.

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