Example _sql_constraints:-

_sql_constraints = [
        ('name_uniq', 'UNIQUE (name)',  'You can not have two users with the same name !')
    ]

'name_uniq' is constraint name give any name.(no need to define)

Example _constraints:-

    def _check_name(self, cr, uid, ids, context=None):
        for val in self.read(cr, uid, ids, ['name'], context=context):
            if val['name']:
                if len(val['name']) < 6:
                    return False
        return True

    _constraints = [
        (_check_name, 'Name must have at least 6 characters.', ['name'])
    ]

Note: Based on the above example in the records name field any duplicate record or name field less than 6 character then remove the records and add the constraints in python restart the server, update the module then only it works.

from odoo.exceptions import ValidationError

import re

@api.constrains('email') 

def validate_mail(self): 

    if self.email: 

          match = re.match('^[_a-z]+[0-9-]*(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,4})$', self.email) 

          if match == None: 

                 raise ValidationError('Not a valid E-mail')

Constraint for emal

@api.constrains('email')   

def _validate_email(self):
        self.email.replace(" ","")        if not re.match(r"[^@]+@[^@]+\.[^@]+", self.email):
            raise exceptions.ValidationError("Please enter valid email address")

_sql_constraints = [       

     ('email_unique',        

    'UNIQUE(email)',        

"The Email must be unique"),   

]

\https://www.youtube.com/watch?v=_5r77hQ43kQ&list=UUZVWw0iLx0pFTzUzvl_XpcA&index=8

Email validation is working fine ,

_sql_constraints = [

('email_uniq', 'unique(email)', 'Email id is unique change your custom email id'),

('phone_uniq','unique(phone)','Phone number is unique so change your phone number ')

]

@api.constrains('email')

def validate_email(self):

for obj in self:

if re.match("^.+\\@(\\[?)[a-zA-Z0-9\\-\\.]+\\.([a-zA-Z]{2,3}|[0-9]{1,3})(\\]?)$", obj.email) == None:

raise ValidationError("Please Provide valid Email Address: %s" % obj.email)

return True