Regexp lookbehind and lineanchors

Question

alelap 2019-9-12

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/480102-regexp-lookbehind-and-lineanchors

编辑： alelap 2019-9-16

Could someone help me to understand why

st = ' a b c';
pattern = '(?<=^\s*)c';
regexp(st,pattern,'lineanchors')
ans =
     []

i.e., does not match (as I expected), while

st2 = [newline,st];
regexp(st2,pattern,'lineanchors')
ans =
     7

i.e., finds a match?

My intent is to match 'c' that is preceded by the beginning of a line and zero or more white character. How should I do?

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Stephen23 2019-9-12

编辑：Stephen23 2019-9-13

Getting an output of 7 seems like a bug to me. Strangely the bug occurs even if the "zero or more matches" character does not even exist in the input string (R2012b):

>> regexp([char(10),st],'(?<=^_*)c','lineanchors') % Underscore is not in st.
ans =
     7
>> regexp([char(10),st],'(?<=^)c','lineanchors') % expected
ans =
     []
>> regexp(st,'(?<=^_*)c','lineanchors') % expected
ans =
     []

What MATLAB version are you using?

You should report this as a bug, giving a link to this thread.

https://www.mathworks.com/support/bug_reports/faq.html

alelap 2019-9-12

编辑：alelap 2019-9-16

R2019a. Reported to Technical Support.

Edit: bug confirmed. Excerpt from Matlab Support's answer:

Indeed the behavior that you observed is indeed a bug in "regexp", which the developers are now aware of, and which might be addressed in some future release.

However, a workaround does exists, which consists in giving up on using the 'lineanchors' option (which makes the "^" and "$" metacharacters match embedded newlines too), and rely on grouping the (absolute) beginning of line "^" and the embedded newline "\n" as two alternatives.

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Answer 1

per isakson 2019-9-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/480102-regexp-lookbehind-and-lineanchors#answer_391658

编辑：per isakson 2019-9-16

"My intent is to match 'c' that is preceded by the beginning of a line and zero or more white character."

In the character array, ' a b c', the character, 'c', is (after the beginning of the line) preceded not only by whitespace but also by the characters 'a' and 'b'. Thus, [] is the expected result. Try

%%
chr = ' a b c';
xpr = '(?<=^[ ab]*)c';
regexp( chr, xpr, 'match', 'lineanchors' )

that returns

ans =
  1×1 cell array
    {'c'}

I fail to understand the behavior of your second example. I expect [], not 7. It's looks like a bug to me.

/R2018b

ADDENDUM

I learned something about the option,'once', the other day. It affects the type of the output. In this case the output is a character row instead of a cell array containing the character row. Thus,

>> regexp( chr, xpr, 'match', 'lineanchors', 'once' )
ans =
    'c'

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alelap 2019-9-13

Thank you. It works. Unfortunately, my example is a simplification of my real scenario where a b c are more complicated expressions and I cannot use this method.