In this short code snippet article I will share the following Regular Expressions (Regex) for validating decimal numbers in TextBox.
1. Regular Expressions (Regex) to match number with decimal precision of exact 1 decimal place.
2. Regular Expressions (Regex) to match number with decimal precision of exact 2 decimal places.
3. Regular Expressions (Regex) to match number with decimal precision of upto 2 decimal places.

Regular Expressions (Regex) to match number with decimal precision of exact 1 decimal place
Regular Expression (Regex)
((\d+)(\.\d{1}))\$

Explanation
This regular expression looks for any number of digits followed by a dot character and ending with one digit character.

Example
<asp:TextBox ID="TextBox1" runat="server" />
<br />
<asp:RegularExpressionValidator ID="Regex1" runat="server" ValidationExpression="((\d+)(\.\d{1}))\$"
ErrorMessage="Please enter valid decimal number with 1 decimal place." ControlToValidate="TextBox1" />

Regular Expressions (Regex) to match number with decimal precision of exact 2 decimal places
Regular Expression (Regex)
((\d+)(\.\d{2}))\$

Explanation
This regular expression looks for any number of digits followed by a dot character and ending with two digit characters.

Example
<asp:TextBox ID="TextBox2" runat="server" />
<br />
<asp:RegularExpressionValidator ID="Regex2" runat="server" ValidationExpression="((\d+)(\.\d{2}))\$"
ErrorMessage="Please enter valid decimal number with 2 decimal places." ControlToValidate="TextBox2" />

Regular Expressions (Regex) to match number with decimal precision of exact 2 decimal places
Regular Expression (Regex)
((\d+)(\.\d{1,2}))\$

Explanation
This regular expression looks for any number of digits followed by a dot character and ending with minimum one digit or maximum two digits.

Example
<asp:TextBox ID="TextBox3" runat="server" />
<br />
<asp:RegularExpressionValidator ID="Regex3" runat="server" ValidationExpression="((\d+)(\.\d{1,2}))\$"
ErrorMessage="Please enter valid decimal number upto 2 decimal places." ControlToValidate="TextBox3" />

Screenshots
Invalid values

Valid values

Demo