Below is the regular expression validator for specifying Minimum and Maximum Password length.
Below are the different configurations of the validator.
 
Alphanumeric with special Characters
Minimum length 7 and Maximum length 10. Characters allowed – a - z  A - Z 0-9 ’@ & # .
<asp:RegularExpressionValidator ID="RegExp1" runat="server"    
ErrorMessage="Password length must be between 7 to 10 characters"
ControlToValidate=" txtPassword "    
ValidationExpression="^[a-zA-Z0-9'@&#.\s]{7,10}$" />
 
Alphanumeric Only
Minimum length 7 and Maximum length 10. Characters allowed – a - z  A - Z 0-9
<asp:RegularExpressionValidator ID="RegExp1" runat="server"    
ErrorMessage="Password length must be between 7 to 10 characters"
ControlToValidate=" txtPassword "    
ValidationExpression="^[a-zA-Z0-9\s]{7,10}$" />
 
Alphabets Only
Minimum length 7 and Maximum length 10. Characters allowed – a - z  A - Z
<asp:RegularExpressionValidator ID="RegExp1" runat="server"    
ErrorMessage="Password length must be between 7 to 10 characters"
ControlToValidate="txtPassword"    
ValidationExpression="^[a-zA-Z]{7,10}$" />
 
Numeric Only
Minimum length 7 and Maximum length 10. Characters allowed – 0 - 9
<asp:RegularExpressionValidator ID="RegExp1" runat="server"    
ErrorMessage="Password length must be between 7 to 10 characters"
ControlToValidate="txtPassword"    
ValidationExpression="^[0-9]{7,10}$" />