To learn more about using regular expressions in your forms, see Introduction to form logic in XLSForm and Introduction to form logic in the Formbuilder.
## Common regex components Regular expressions in KoboToolbox are written inside the `regex()` function. For example, `regex(., '^[0-9]{10}$')` restricts an input to exactly 10 digits. The following tables describe commonly used regex elements. ### Anchors and grouping | Regex | Description | |:---|:---| |^ | Matches the start of a string |
| $ | Matches the end of a string |
| ( ) | Groups characters together |
| \| | Matches one pattern or another (logical OR) |
### Character classes
| Regex | Description |
|:---|:---|
| [abc] | Matches a, b, or c |
| [a-z] | Matches any lowercase letter |
| [A-Z] | Matches any uppercase letter |
| [0-9] | Matches any digit from 0 to 9 |
| [a-zA-Z0-9] | Matches letters or digits |
| \[^abc] | Matches any character except a, b, or c |
| \[^A-Z] | Matches any character except uppercase letters |
### Special character shortcuts
| Regex | Description |
|:---|:---|
| \d | Matches any digit (same as [0-9]) |
| \D | Matches any non-digit |
| \w | Matches any word character (letters, digits, _) |
| \W | Matches any non-word character |
| \s | Matches a space or tab |
| \b | Matches a word boundary |
| \\. | Matches a literal dot (.) |
| \\@ | Matches a literal @ |
| \\$ | Matches a literal $ |
| `\\` | Matches a literal backslash (\\) |
| \number | Refers to a previously matched group |
### Quantifiers
| Regex | Description |
|:---|:---|
| ? | Matches zero or one occurrence |
| * | Matches zero or more occurrences |
| + | Matches one or more occurrences |
| {x} | Matches exactly x occurrences |
| {x,} | Matches at least x occurrences |
| {x,y} | Matches between x and y occurrences |
## Common examples
The following examples can be used as [constraints](https://support.kobotoolbox.org/constraints_xls.html) or [validation criteria](https://support.kobotoolbox.org/validation_criteria.html) in your forms.
### Common regex expressions with numbers
| Regex | Description |
|:---|:---|
| regex(., '^\d+$') | Limit input to numbers |
| regex(., '^\d{2}$') | Limit input to 2 digits |
| regex(., '^\d{2,4}$') | Limit input to 2-4 digits |
| regex(., '^\d{10}$') | Limit input to 10 digits |
| regex(., '^[1-9][0-9]?$|^100$') | Limit input to a number between 1 and 100 (e.g., for a percentage) |
| regex(., '^[1-9][0-9]{8}$') | Limit input to 9 digits, first digit cannot be 0 |
| regex(., '^\d{2}\\.\d{3}$') | Limit input to number formatted as 12.345 |
| regex(., '^(\d{10}|\d{13}|\d{17})$') | Limit input to 10, 13, or 17 digits |
| regex(., '^(12|345)$') | Input must be 12 or 345 |
### Common regex expressions with letters
| Regex | Description |
|:---|:---|
| regex(., '^\D*$') | Limit input to letters, spaces, and symbols (no numbers) |
| regex(., '^[A-Za-z]+$') | Limit input to letters |
| regex(., '^[A-Za-z\s]+$') | Limit input to letters and spaces |
| regex(., '^[a-z]{1,6}$') | Limit input to 1–6 lowercase letters |
| regex(., '^[A-Z]{1,10}$') | Limit input to 1–10 uppercase letters |
| regex(., '^(Apple|Orange|Banana)$') | Input must match one word from the list |
| regex(., '^colou?r$') | Allows color or colour |
| regex(., '^[A-Z\s]+$') | Limit input to uppercase letters and spaces only |
| regex(., '^[a-z_]+$') | Limit input to lowercase letters and underscores (_) |
### Common regex expressions with letters and numbers
| Regex | Description |
|:---|:---|
| regex(., '^\w{3}$') | Limit input to exactly 3 letters, digits, or underscores (_) |
| regex(., '^[A-Z]{1}[0-9]{8}$') | Limit input to one letter and eight numbers |
| regex(., '^CAR-PRC-2020-\d{4}$') | Limit input to a specific ID format |
| regex(., '^\W*(\w+\b\W*){3}$') | Limit input to exactly 3 words |
| regex(., '^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9]+(-[a-zA-Z0-9]+)*(\\\.[a-zA-Z0-9]+(-[a-zA-Z0-9]+)*)*\\\.[a-zA-Z]{2,}$') | Limit input to common email format |
For additional help building and testing patterns, visit: http://www.regexr.com/