* [exprtk attribute](#exprtk-attribute)

The `exprtk` attribute validates a menu based on a string expression algorithm. The expression must be specified as a mathematical expression and the result must evaluates to `true` or `false` such as `4 == 5` or `10 > 3`.

If `exprtk` attribute is specified, the expression must evaluates to `true` for the validation to be successful. The `exprtk` attribute does not support multiple expressions but logical `and` and `or` operators can be use to group expressions.

If `exprtk` attribute is not specified, then the validation is successful.

The attribute supports the following operators:

* Basic operators: `+`, `-`, `*`, `/`, `%`, `^`

* Equalities & Inequalities: `=`, `==`, `<>`, `!=`, `<`, `<=`, `>`, `>=`

* Logic operators: `and`, `not`, `or`, `xor`, `true`, `false`

* String operators: `in`, `like`, `ilike`, []

Strings may be comprised of any combination of letters, digits special characters including (~!@#$%^&*()[]|=+ ,./?<>;:"``~_) or hexadecimal escaped sequences (eg: \0x30) and must be enclosed with single-quotes.

eg: `'Frankly my dear, \0x49 do n0t give a damn!'`

The `exprtk` attribute allows advanced menu validation. The following table show useful expression examples:

| Use cases | Expression | Meaning |

|------------------------------------------------------------------|-----------------------------------|----------------------------------------------------------------------------------------------------------------------------|

| Test a property for a numeric value | ${value} == 6 | Evaluates to true when property `value` is set to numeric value `6`. |

| Test a property for a string value. | '${name}' == 'John' | Evaluates to true when property `name` is set to string value `John`. |

| Set a menu visible based on how many file are selected | ${selection.count} == 3 | Evaluates to true when user clicked on exactly 3 elements. |

| Set a menu visible based on a state machine | '${myapp.state}' == 'PAUSED' | Evaluates to true when application's state is `PAUSED`. |

| Set a menu *invisible* when it was selected 3 times. | ${myapp.runs} <= 2 | Evaluates to true when property `myapp.runs` is lower or equals to `2`. |

| Set a menu visible by filename length. | '${selection.filename}'[] == 9 | Evaluates to true when user clicked on a file whose filename<br>(including file extension) is exactly 9 characters. |

| Combine expressions with `and` and `or` logic. | ${foo} == 2 or ${bar} >= 5 | Evaluates to true when property `foo` is set to `2` *or*<br>when property `bar` is set to a value greater or equal to `5`. |

| Set a menu visible if user selection contains a specific string. | 'abc' in '${selection.path}' | Evaluates to true when user clicked on a file that contains the string `abc`. |

| Set a menu visible if user selection matches a string pattern. | '${selection.path}' ilike '*.exe' | Evaluates to true when user clicked on a file with `exe` extension. |

**Note:**

The `exprtk` attribute uses the *exprtk library* to parse the expression. For more details and supported expressions, see the exprtk documentation on the [official github page](https://github.com/ArashPartow/exprtk) or the [library website](http://www.partow.net/programming/exprtk/index.html).

| selection.count | Matches the number of clicked elements (files and directories). |

| selection.files.count | Matches the number of clicked files. |

| selection.directories.count | Matches the number of clicked directories. |

Notes:

* Properties `selection.drive.letter` and `selection.drive.path` are empty when all selected files are from a network share.

* Properties `selection.count`, `selection.files.count` and `selection.directories.count` are not multi-selection-based properties. They are defined as a single value whether a single or multiple elements are selected.