Add regex replace and function call docs

docs/content/main/expression_language.md

@@ -34,4 +34,7 @@ The expression language supports:
     - strings can contain other expressions again: `'foo {{some_variable}} bar'`
 - json access (`object.field`, `array[12]`, `object["field"]`)
     - for this, the object/array value needs to refer to a variable that contains a valid json string.
+- some function calls:
+    - `round(number, decimal_digits)`: Round a number to the given amount of decimals
+    - `replace(string, regex, replacement)`: Replace matches of a given regex in a string
 

src/value/attr_value/attr_value_expr.rs

@@ -230,6 +230,15 @@ fn call_expr_function(name: &str, args: Vec<PrimVal>) -> Result<PrimVal> {
             }
             _ => Err(anyhow!("Incorrect number of arguments given to {}", name)),
         },
+        "replace" => match args.as_slice() {
+            [string, pattern, replacement] => {
+                let string = string.as_string()?;
+                let pattern = regex::Regex::new(&pattern.as_string()?)?;
+                let replacement = replacement.as_string()?;
+                Ok(PrimVal::from(pattern.replace_all(&string, replacement.replace("$", "$$").replace("\\", "$")).into_owned()))
+            }
+            _ => Err(anyhow!("Incorrect number of arguments given to {}", name)),
+        },
         _ => Err(anyhow!("Unknown function {}", name)),
     }
 }