> For the complete documentation index, see [llms.txt](https://php-fhir-tools.ardenexal.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://php-fhir-tools.ardenexal.net/fhirpath/functions/string.md).

# String

String functions operate on a single-item String input unless noted otherwise.

| Function                              | Description                                                           | Example                                 |
| ------------------------------------- | --------------------------------------------------------------------- | --------------------------------------- |
| `substring(start [, length])`         | Returns the substring from `start` (0-based) for `length` characters. | `'abcdef'.substring(0, 3)` → `'abc'`    |
| `length()`                            | Returns the character count as an Integer.                            | `'abc'.length()` → `3`                  |
| `startsWith(prefix)`                  | True when the string begins with `prefix`.                            | `'abc'.startsWith('ab')`                |
| `endsWith(suffix)`                    | True when the string ends with `suffix`.                              | `'abc'.endsWith('bc')`                  |
| `contains(substring)`                 | True when the string contains `substring`.                            | `'abc'.contains('b')`                   |
| `indexOf(substring)`                  | 0-based index of the first match, or `-1`.                            | `'abc'.indexOf('b')` → `1`              |
| `upper()`                             | Converts to upper case.                                               | `'abc'.upper()` → `'ABC'`               |
| `lower()`                             | Converts to lower case.                                               | `'ABC'.lower()` → `'abc'`               |
| `replace(pattern, substitution)`      | Replaces all plain-string occurrences of `pattern`.                   | `'abc'.replace('b', 'x')` → `'axc'`     |
| `replaceMatches(regex, substitution)` | Regex-based replace.                                                  | `'abc123'.replaceMatches('[0-9]', 'x')` |
| `matches(regex)`                      | True when the regex matches anywhere in the string.                   | `'abc'.matches('[a-z]+')`               |
| `matchesFull(regex)`                  | True when the regex matches the entire string.                        | `'abc'.matchesFull('[a-z]+')`           |
| `trim()`                              | Removes leading and trailing whitespace.                              | `' abc '.trim()` → `'abc'`              |
| `split(separator)`                    | Splits the string into a collection.                                  | `'a,b,c'.split(',')`                    |
| `toChars()`                           | Splits the string into a collection of single characters.             | `'abc'.toChars()`                       |
| `join([separator])`                   | Joins a collection of strings into one string.                        | `('a' \| 'b').join(',')` → `'a,b'`      |
| `encode(format)`                      | Encodes a string (`hex`, `base64`, `urlbase64`).                      | `'abc'.encode('base64')`                |
| `decode(format)`                      | Decodes a string using the given format.                              | `'YWJj'.decode('base64')` → `'abc'`     |
| `escape(target)`                      | Escapes a string for `html` or `json`.                                | `'<a>'.escape('html')`                  |
| `unescape(target)`                    | Reverses `escape()` for `html` or `json`.                             | `'&lt;a&gt;'.unescape('html')`          |


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