Level 3: Real-world validation · Exercise 45 of 88 · Free

Decimal numbers

A regex exercise from Regexdrill. Solve it with a pattern box with live highlighting in your browser, checked against hidden test strings, and it is checked the moment you submit.

The task

The whole line must match. Match a number: an optional minus sign, then 0 or a whole part with no leading zeros, then optionally a period followed by one or more digits. So -12.5, 0.25 and 7 match, but 007, .5 and 3. do not.

What this exercise teaches

-?(?:0|[1-9]\d*)(?:\.\d+)? handles each rule in turn. The alternation 0|[1-9]\d* is the standard way to forbid leading zeros while still allowing a lone 0, and making the period part of the optional group rules out a trailing period.

A hint if you get stuck

The whole part is an alternation: either a single 0, or a digit from 1 to 9 followed by any digits. The decimal part is an optional group.