Configuration via {{slapd.conf}}(5) would look like:
> overlay constraint
-> constraint_attribute mail regex ^[:alnum:]+@mydomain.com$
+> constraint_attribute mail regex ^[[:alnum:]]+@mydomain.com$
> constraint_attribute title uri
> ldap:///dc=catalog,dc=example,dc=com?title?sub?(objectClass=titleCatalog)
> objectClass: olcOverlayConfig
> objectClass: olcConstraintConfig
> olcOverlay: constraint
-> olcConstraintAttribute: mail regex ^[:alnum:]+@mydomain.com$
+> olcConstraintAttribute: mail regex ^[[:alnum:]]+@mydomain.com$
> olcConstraintAttribute: title uri ldap:///dc=catalog,dc=example,dc=com?title?sub?(objectClass=titleCatalog)
overlay constraint
constraint_attribute jpegPhoto size 131072
constraint_attribute userPassword count 3
-constraint_attribute mail regex ^[:alnum:]+@mydomain.com$
+constraint_attribute mail regex ^[[:alnum:]]+@mydomain.com$
constraint_attribute title uri
ldap:///dc=catalog,dc=example,dc=com?title?sub?(objectClass=titleCatalog)
constraint_attribute cn,sn,givenName set