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How do I write a regular expression that consists of the characters {x,y} but must start and end with the same letter? For example:
xyyyxyxyxyxyxy
252
The caret ^ and dollar $ characters have special meaning in a regexp. They are called “anchors”. The caret ^ matches at the beginning of the text, and the dollar $ – at the end.
Put brackets ( [ ] ) in the pattern string, and inside the brackets put the lowest and highest characters in the range, separated by a hyphen ( – ). Any single character within the range makes a successful match.
End of String or Before Ending Newline: \Z The \Z anchor specifies that a match must occur at the end of the input string, or before \n at the end of the input string.
The ^ and $ match the beginning and ending of the input string, respectively. The \s (lowercase s ) matches a whitespace (blank, tab \t , and newline \r or \n ). On the other hand, the \S+ (uppercase S ) matches anything that is NOT matched by \s , i.e., non-whitespace.
This should work for you:
^(x|y).*\1$
This regex will match a string that starts and ends with the same letter (as the post title suggests), but does not limit the string to contain only x and y characters. It will match any strings, starting and ending with the same letters specified in the parenthesis.
It will match strings consisting of {x,y} characters, starting and ending with the same letter: (as the OP specified.)
xyyyxyx
yxyxyxy
zxyxyxz
xyxyxyy
But it will also match strings with any characters in between (not limited to only x and y):
xgjyhdtfx
yjsaudgty
xuhgrey
yudgfsx
yaaay
Working regex example:
https://regex101.com/r/TER7zI/1
82
This regex works:
^([xy])[xy]*\1$|^[xy]$
I tested it on regexr with
xyyyxyx
yxyxyxy
x
y
xyyyxyy
yxyxyxx
xyzyxx
z
and it only matched the first four.
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