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I would like to write a regular expression that matches a text if it contains a string ONLY once. The text must contain <scr> only once. Here are some examples:
hello-<scr>Keephello-endofstring //ok; <scr> occurs once
test-<scr>bla<scr>bla-end //NOT ok; <scr> occurs 2 times
hello-Keephello-end //NOT ok; <scr> doesn't occur
I tried with the following regex:
((?:(?<!<scr>).)*<scr>(?:(?!<scr>).)*)
The first negative lookbehind ensures that <scr> doesn't occur.
Than <scr> must follow.
After this a negative lookahead ensures that no more <scr> follow.
It does not work.
I would like to know how this can be done with regex? (with explanation)
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$ means "Match the end of the string" (the position after the last character in the string). Both are called anchors and ensure that the entire string is matched instead of just a substring.
[] denotes a character class. () denotes a capturing group. [a-z0-9] -- One character that is in the range of a-z OR 0-9. (a-z0-9) -- Explicit capture of a-z0-9 .
Basically (0+1)* mathes any sequence of ones and zeroes. So, in your example (0+1)*1(0+1)* should match any sequence that has 1. It would not match 000 , but it would match 010 , 1 , 111 etc. (0+1) means 0 OR 1. 1* means any number of ones.
To check if text contains some substring only once, you need to check match all characters that do not constitute <scr>, then match <scr>, and use a negative look-ahead to check if there is no <scr> further, and consume all characters. Also, line/string boundaries ^/$ are a must:
^(?:(?!<scr>).)*<scr>(?!.*<scr>).*$
See demo
EXPLANATION:
^ - Start of line (as m multiline option is ON)(?:(?!<scr>).)* - A non-capturing group to match each character (not a newline - for that, you need to also add s singleline option) that is not preceded with <scr>
<scr> - Our literal <scr>
(?!.*<scr>) - The negative lookbehind checking that we do not have <scr> any more .*$ - Subpattern matching the rest of the characters to the end of the line.
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