I was wondering if it is possible to simplify the following regular expression

^(?!.*([,\._\- ]).*\1)(?!.*[',\._\- ]{2})(?!.*(['])(?:.*\2){2})[',\._\- \p{L}] $

Regex Demo

Constraints

1. All of these characters can appear a maximum of one time each: _-.
   • The last character is a white space.
2. This character can appear a maximum of two times each: '
3. All special characters here can not appear consecutively.

Details

1. (?!.*([,\._\- ]).*\1) - There cannot be 2 or more occurrences of any one character in _-.
2. (?!.*[',\._\- ]{2}) - No char in the special char group can appear consecutively.
3. (?!.*(['])(?:.*\2){2}) - Single quote special char cannot appear 3 or more times.

CodePudding user response：

You might write the pattern using a single capture group in combination with negated character classes making use of contrast

You don' have to create a capture group ([']) with a backreference \2 to repeat a single quote 3 times, but you can just repeat that char 3 times.

As there do not seem to be ' at the start or at the end, you can use a repeating pattern \p{L} (?:[',._ -]\p{L} )* to not have consecutive chars that you don't want.

Note that you don't have to escape the ' and _ in the character class, and you can move the - to the end so that you don't have to escape it.

^(?![^,._ \n-]*([,._ -]).*\1)(?!(?:[^'\n]*'){3})\p{L} (?:[',._ -]\p{L} )*$

Explanation

• ^ Start of string
• (?![^,._ \n-]*([,._ -]).*\1) Assert not 2 of the same chars [,._ -]
• (?!(?:[^'\n]*'){3}) Assert not 3 times '
• \p{L} Match 1 times any kind of letter
• (?:[',._ -]\p{L} )* Optionally repeat one of [',._ -] and again 1 times any kind of letter
• $ End of string

Regex demo