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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