I am struggling with a regular expression for detecting if a special character (hashtag #) is included exactly once in a string. Everything before and after the hashtag is unknown and can be any symbol (including whitespace and special characters).
Examples:
# matches
## doesnt match
a12bc#1def65 matches
a12bc##1def65 doesnt match
a234341#1212 matches
a23$. 4341#1 ._21&2 matches
#a23$. 4341#1 ._21&2# doesnt match
I think that would be quiet simple (I am not really good in regex) so I tried
#{1}
Surprisingly this does not work as I expected. This regex matches also the string ##.
Then I tried some stuff like #{1}\b and (#{1}\b|[^#{2}]). But everytime either the string # dont match or ## do match the regex.
CodePudding user response：
This pattern #{1}\b can be written as #\b and matches # only if followed by a word boundary. This does not take a number of occurrences into account for a whole line.
This pattern (#{1}\b|[^#{2}]) has the same as before, followed by an alternation | and negated character class [^#{2}] that can match a single char other than chars # { 2 }
If you don't want to match newlines, and match the whole string:
^[^\r\n#]*#[^\r\n#]*$
^Start of string[^\r\n#]*Optionally match any char except a newline or##Match the#char[^\r\n#]*Optionally match any char except a newline or#$End of string