How do I create a Custom Mask?

Modified on Fri, 19 Dec at 8:41 AM

Custom Mask Expressions in Droplet let you define exactly how a user’s input should be structured in an Input (Single Line) component. By restricting allowable characters and formatting patterns, you can ensure cleaner, more consistent data in your forms.

When Should I Use a Custom Mask?

Masks in Droplet help ensure consistent, valid data entry. For instance, if you want a user to enter a specific number of digits (like a phone number) or strictly allow letters only (like a middle initial), custom masks let you define rules that enforce the exact format and type of characters entered.

Typically, you’d use a custom mask when:

You want to limit or validate input character-by-character in a single-line text field.

None of the Built-in Masks (like phone number, SSN, ZIP code) fit your use-case.

In essence, a custom mask is your go-to tool when you want to control exactly how input gets typed.

Note: masks do not validate the field value, only restrict user input to guide them. To learn more about checking field values for accuracy, see this article on validation rules.

/9
Numeric character placeholder (0-9)

Example Use

/9/9/9/9

Accepts exactly 4 digits (like a year).

/a
Alphabetic character placeholder (A-Z, a-z)

Example Use

Accepts exactly 1 letter (like a middle initial).

/*
Alpha-numeric placeholder (letters or digits)

Example Use

Accepts exactly 15 alpha-numeric characters (e.g., Batch ID).

/l
Lowercase character placeholder

Example Use

/l/l

Accepts exactly 2 lowercase letters.

/u
Uppercase character placeholder

Example Use

/u/u/u

Accepts exactly 3 uppercase letters.

/?
Any character placeholder (a wildcard)

Example Use

/?/?-/?/?/?

Accepts exactly 2 characters, then a dash, then exactly 3 characters. No restriction on letters, digits, etc.

Literal Characters
Certain characters appear exactly as typed — they are not placeholders. These can be letters, numbers, punctuation, parentheses, dashes, dots, etc.

Examples:
3, G, -, (, ), ., :, /

Example Use

ABC-/9/9/9

This mask accepts 3 numeric digits, but must begin with ABC- first (e.g. ABC-123 or ABC-456).

User-Defined Sets
Lets you specify exactly which characters are allowed. Use square brackets to define your set.

Example Use

/[a-d123]

Accepts only letters a, b, c, d and digits 1, 2, 3 in a single slot.

Ranges
Specify exact or variable lengths using parentheses. You can do exact or min:max.

Example Uses

/(3)9

Exactly three digits.

/(2:3)9

Accepts between 2 and 3 digits. Perfect for optional digit lengths (e.g., 99 or 999).

NOT Operator (!)
Exclude certain characters or sets with !

2 Exceptions
Cannot use the NOT operator with ranges or with the wildcard (for example, /!? is invalid).

Example Use

/!9/!9/!9

Three characters, each must not be a digit. Could be letters, punctuation, etc. (e.g., XYZ, !!!, but not 123).