What causes "{name} must be a decimal number" in Semantic-Org/Semantic-UI?

If negatives/leading-dot are valid, switch to the `number` rule (regex `/^-?\d*(\.\d+)?$/`).

{name} must be a decimal number

Error message

{name} must be a decimal number

What it means

Default prompt for the Semantic UI Form `decimal` rule (settings.rules.decimal, form.js:1412). It tests the value against `settings.regExp.decimal` = `/^\d+\.?\d*$/` (form.js:1239). Note the regex rejects negative numbers and leading-dot decimals: `-1.5`, `.5`, and `-.5` all FAIL despite being valid decimals.

Source

Thrown at src/definitions/behaviors/form.js:1260

    flags   : /^\/(.*)\/(.*)?/,
    integer : /^\-?\d+$/,
    number  : /^\-?\d*(\.\d+)?$/,
    url     : /(https?:\/\/(?:www\.|(?!www))[^\s\.]+\.[^\s]{2,}|www\.[^\s]+\.[^\s]{2,})/i
  },

  text: {
    unspecifiedRule  : 'Please enter a valid value',
    unspecifiedField : 'This field'
  },

  prompt: {
    empty                : '{name} must have a value',
    checked              : '{name} must be checked',
    email                : '{name} must be a valid e-mail',
    url                  : '{name} must be a valid url',
    regExp               : '{name} is not formatted correctly',
    integer              : '{name} must be an integer',
    decimal              : '{name} must be a decimal number',
    number               : '{name} must be set to a number',
    is                   : '{name} must be "{ruleValue}"',
    isExactly            : '{name} must be exactly "{ruleValue}"',
    not                  : '{name} cannot be set to "{ruleValue}"',
    notExactly           : '{name} cannot be set to exactly "{ruleValue}"',
    contain              : '{name} must contain "{ruleValue}"',
    containExactly       : '{name} must contain exactly "{ruleValue}"',
    doesntContain        : '{name} cannot contain  "{ruleValue}"',
    doesntContainExactly : '{name} cannot contain exactly "{ruleValue}"',
    minLength            : '{name} must be at least {ruleValue} characters',
    length               : '{name} must be at least {ruleValue} characters',
    exactLength          : '{name} must be exactly {ruleValue} characters',
    maxLength            : '{name} cannot be longer than {ruleValue} characters',
    match                : '{name} must match {ruleValue} field',
    different            : '{name} must have a different value than {ruleValue} field',
    creditCard           : '{name} must be a valid credit card number',
    minCount             : '{name} must have at least {ruleValue} choices',
    exactCount           : '{name} must have exactly {ruleValue} choices',

View on GitHub (pinned to 597843ab84)

Solutions

If negatives/leading-dot are valid, switch to the `number` rule (regex `/^-?\d*(\.\d+)?$/`).
Strip a leading sign before validating, or use a custom `regExp` rule like `regExp[/^-?\d+\.?\d*$/]`.
Replace comma decimals with a dot in a pre-submit handler.
Provide a custom `prompt` clarifying the expected format.

Example fix

// before - rejects -2.5 because regExp.decimal has no optional sign
rules: [{ type: 'decimal' }]

// after - accept signed decimals
rules: [{ type: 'regExp', value: /^-?\d+\.?\d*$/, prompt: 'Enter a decimal number' }]

Defensive patterns

Strategy: validation

Validate before calling

// Accept signed decimals (the built-in regExp.decimal does not)
const decimalOk = /^-?\d+\.?\d*$/.test($field.val().replace(',', '.'));
if (!decimalOk) { /* warn */ }

Type guard

const isSignedDecimal = (v) => /^-?\d+\.?\d*$/.test(String(v).trim());

Prevention

Remember regExp.decimal rejects negatives and leading dots — switch to a custom regex when needed.
Normalize comma decimals to dots before validation.
Prefer the number rule when sign support is required.

When it happens

Trigger: `{type:'decimal'}` with input `-2.5`, `.75`, `abc`, `1,5` (comma decimal), or an empty string.

Common situations: Fields that must accept negative amounts or currency; locales using comma decimals; values typed with a leading dot. The regex quirk is the most common surprise.

Related errors

AI-assisted analysis of Semantic-Org/Semantic-UI@597843ab84 (2026-08-13). Data as JSON: /api/errors/2da18ee49a18e565. Report an issue: GitHub.