Already have a working regexp ^[a-zA-Z0-9 ]{1,11}$ but recently got an update in validation: there should be all old validations and new one symbol _ which should appear in whole string exactly once.
Strings must only match:
_name
name_
name_name
validation should fail for:
_name_
__name
etc.
tried a few approaches ^[a-zA-Z0-9_ ]{1,11}$ all characters may be _
^(?)[a-zA-Z0-9 ]{1,11}(?)$ validates
alpha_
_alpha
_alpha_ (shouldn't be valid)
but not this one:
alpha_name
CodePudding user response：
You need to use
^(?=.{1,11}$)[a-zA-Z0-9 ]*(?:_[a-zA-Z0-9 ]*)?$
See the regex demo. Details:
^- start of string(?=.{1,11}$)- a positive lookahead that requires one to eleven chars other than line break chars till end of string[a-zA-Z0-9 ]*- zero or more letters, digits or spaces(?:_[a-zA-Z0-9 ]*)?- an optional occurrence of_and zero or more letters, digits or spaces$- end of string.
CodePudding user response：
Try this:
^(?=.{1,11}$)([a-zA-Z0-9 ]*)_?(?1)$