## Regular expression for numbers

30 Dec 2023

It's easy to find a positive integer number with regular expressions:

[0-9]+

This regex means digits from 0 to 9, repeated one or more times. However, **numbers starting with zero** are treated as octal in many programming languages, so you may wish to avoid matching them:

[1-9][0-9]*

This regular expression matches any positive integer number starting with a non-zero digit. If you also need to match zero, you can include it as another branch:

[1-9][0-9]*|0

To also accomodate **negative integer numbers,** you can allow a minus sign before the digits:

-?[1-9][0-9]*|0

Sometimes it's necessary to allow a plus sign as well:

[-+]?[1-9][0-9]*|0

The previous regexes searched the input string for a number. If you need to match **a number only** discarding anything else, you can add the `^`

anchor to match the beginning of the string and the `$`

anchor to match the end:

^(-?[1-9][0-9]*|0)$

Parentheses are necessary here; without them, the `^`

anchor would apply only to the first branch. Another variation of the same regex avoids finding numbers that are part of words, such as `600px`

or `x64`

:

\b(-?[1-9][0-9]*|0)\b

Things get more complicated if you need to match **a fractional number**:

\b-?(?:[1-9][0-9]*(?:\.[0-9]+)?|\.[0-9]+|0)\b

Let's break down this regular expression:

- The first branch
`[1-9][0-9]*(?:\.[0-9]+)?`

matches an integer number starting with a non-zero digit, then an optional fractional part. - The second branch
`\.[0-9]+`

matches fractional numbers starting with a dot, for example,`.5`

is another way to write`0.5`

. - The third branch matches zero. Note that both positive and negative zeros are possible in floating-point numbers.

For floating-point numbers with an exponent, such as `5.2777e+231`

, please use:

\b-?(?:[1-9][0-9]*(?:\.[0-9]+)?|\.[0-9]+|0)(?:[eE][+-]?[0-9]+)?\b

Many programming languages support **hexadecimal numbers** starting with `0x`

. Here is a regular expression to match them:

0x[0-9a-fA-F]+

Finally, here is a comprehensive regular expression to match floating-point, integer decimal, or hexadecimal numbers:

\b-?(?:[1-9][0-9]*(?:\.[0-9]+)?|\.[0-9]+|0(?:x[0-9a-fA-F]+)?)(?:[eE][+-]?[0-9]+)?\b

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