I'm trying to write a regex similar to
^[a-zA-Z.{0,1},{0,1}'{0,1}]*
However, this can accept more than one comma, period, and apostrophes.
For example "String.,'" is allowed but "str.,..,,,'''" should not be allowed.
Use
^(?!.*([.,']).*\1)[a-zA-Z.,']*$
See regex proof.
EXPLANATION
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^ the beginning of the string
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(?! look ahead to see if there is not:
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.* any character except \n (0 or more times
(matching the most amount possible))
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( group and capture to \1:
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[.,'] any character of: '.', ',', '''
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) end of \1
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.* any character except \n (0 or more times
(matching the most amount possible))
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\1 what was matched by capture \1
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) end of look-ahead
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[a-zA-Z.,']* any character of: 'a' to 'z', 'A' to 'Z',
'.', ',', ''' (0 or more times (matching
the most amount possible))
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$ before an optional \n, and the end of the
string