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IsNumber

Function IsNumber( const S : string) : Boolean

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := IsNumber('-12.50');
end;

Usage​

IsNumber checks a non-empty lexical number made from digits, one period and an optional leading sign.

Parameters

NameTypeDescription
Sstring, constText inspected exactly as supplied. The function does not trim it.

Returns

False for an empty string, an unrecognised inspected character, a second period or a sign after the first position. Otherwise True.

Behaviour

Conventional forms such as 12, -12, +12.5, .5 and 5. pass. The decimal separator is always a literal period; machine and user locale settings are ignored.

Errors

The lexical scan has no normal exception path. Subsequent conversion can still raise for sign-only/period-only text, range overflow or a mismatched decimal-format contract.

Additional Technical Info​

IsNumber checks a small, locale-independent lexical shape: ASCII digits, at most one period, and + or - only in the first character. It does not parse the text or guarantee that a numeric conversion will succeed.

The example returns True. It is source-reviewed and is not executed by the documentation workflow.

Implementation

The Velox routine scans a PChar until the first null character. It permits characters in ['0'..'9', '.', '-', '+'], tracks whether a period has already appeared, and tracks whether it is still at the first character for sign placement.

It does not track whether any digit appeared.

Edge cases and quirks

  • Because no digit is required, ., +, -, +., -. and similar forms return True even though normal numeric conversion can reject them.
  • A plus or minus is accepted only as the first character. Exponent forms such as 1E-3 are rejected because E is not allowed.
  • A second period is rejected. Commas, spaces and non-ASCII digits are rejected.
  • The function performs no range, precision or overflow check.
  • An embedded #0 ends scanning and hides later characters. A nonempty string starting with #0 can return True without supplying a digit.
  • True means only that the inspected prefix fits this state machine; it is not a successful parse result.

Side effects

None.

Performance and concurrency

Time is linear in the inspected prefix. The routine allocates no transformed string and uses no shared mutable state.

Remarks

Follow this filter with the conversion function and explicit TFormatSettings/domain range required by the source contract. If the goal is simply to know whether conversion is possible, a TryStrToFloat-style operation under explicit format settings is a stronger test.

Related entries

Created 2026-07-15