uCalc API Version: 2.1.3-preview.2 Released: 6/17/2026

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uCalc API Preview Release Notice:The documentation describes the intended behavior of the API. The current preview build contains incomplete features, unoptimized performance, and is subject to breaking changes.

Executing Logic in Replacements

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Learn to execute expressions and call functions within replacement strings using the powerful {@Eval} and {@@Eval} directives.

Remarks

🧠 Executing Logic in Replacements

Traditional find-and-replace tools are typically limited to static text and simple backreferences ($1, $2). uCalc's Transformer elevates this by allowing you to execute complex logic, perform calculations, and call functions directly within the replacement string. This turns a simple text substitution into a dynamic, intelligent transformation.

The engine for this capability is a set of special pattern methods that evaluate expressions at different stages of the transformation process.

The Core Tools: {@Eval}, {@@Eval}, and {@Exec}

1. {@Eval}: The Workhorse for Dynamic Values

The {@Eval} directive is the most common tool for executing logic. It evaluates a fixed expression using dynamic values captured from the current match. For performance, the expression is parsed only once when the rule is created.

To use a variable captured in your pattern (e.g., {qty}), simply refer to it by name inside the Eval block without curly braces.

// The expression 'Double(qty) * Double(price)' is fixed.// The values for 'qty' and 'price' change with each match.t.FromTo("Qty: {@Number:qty}, Price: {@Number:price}",          "{@Self}, Total: {@Eval: Double(qty) * Double(price)}");

2. {@@Eval}: The Power Tool for Dynamic Formulas

The {@@Eval} directive (with a double @@) is for meta-programming scenarios where the expression itself is captured from the source text. It parses and evaluates its content every single time a match is found.

This allows you to create a transformer that can solve mathematical problems embedded in a string.

// The variable {formula} captures the text "10 * (5-2)".// {@@Eval} then executes that captured text as an expression.t.FromTo("solve({formula})", "{@@Eval: formula}");

3. {@Exec}: For Side Effects Only

The {@Exec} directive is a "void" version of {@Eval}. It executes an expression but does not insert any text into the replacement string. It is used exclusively for side effects, such as incrementing a counter, logging information, or setting a Variable that will be used by another rule.

// This rule finds a word but inserts nothing into the text as a replacement.// Its only purpose is to increment the 'word_count' variable.t.FromTo("{@Alpha}", "{@Self}{@Exec: word_count++}");

⚠️ Important: Type Conversion

All variables captured from a pattern, even those from {@Number:val}, are text strings. Before you can use them in a mathematical calculation inside an {@Eval} block, you must explicitly convert them to a numeric type using functions like Double() or Int().

  • Correct: {@Eval: Double(val) * 2}
  • Incorrect: {@Eval: val * 2} (if val is 123, this would result in 123123).

💡 Why uCalc? (Comparative Analysis)

In most programming environments, performing logic during a regex replacement requires writing a separate callback function (like a MatchEvaluator in C#). This separates the logic from the pattern and can be verbose.

C# MatchEvaluator Approach:

string ProcessMatch(Match m){    int qty = int.Parse(m.Groups["qty"].Value);    double price = double.Parse(m.Groups["price"].Value);    return m.Value + ", Total: " + (qty * price);}Regex.Replace(input, @"Qty: (?<qty>\d+), Price: (?<price>[\d\.]+)", ProcessMatch);

The uCalc Advantage:uCalc's declarative approach allows the logic to live directly and cleanly within the replacement string, resulting in more concise and readable rules.

t.FromTo("Qty: {@Number:qty}, Price: {@Number:price}",          "{@Self}, Total: {@Eval: Double(qty) * Double(price)}");

Examples

A succinct example that performs a simple unit conversion from inches to centimeters using `{@Eval}`.
				
					using uCalcSoftware;

var uc = new uCalc();
var t = new uCalc.Transformer();

// Capture a number followed by 'in' and convert it.
// Note: `len` is a string, so we must use Double(len) for the calculation.
t.FromTo("{@Number:len}in", "{@Eval: Double(len) * 2.54}cm");

Console.WriteLine(t.Transform("The board is 10in long."));
				
			
The board is 25.4cm long.
				
					#include <iostream>
#include "uCalc.h"

using namespace std;
using namespace uCalcSoftware;

int main() {
   uCalc uc;
   uCalc::Transformer t;

   // Capture a number followed by 'in' and convert it.
   // Note: `len` is a string, so we must use Double(len) for the calculation.
   t.FromTo("{@Number:len}in", "{@Eval: Double(len) * 2.54}cm");

   cout << t.Transform("The board is 10in long.") << endl;
}
				
			
The board is 25.4cm long.
				
					Imports System
Imports uCalcSoftware
Public Module Program
   Public Sub Main()
      Dim uc As New uCalc()
      Dim t As New uCalc.Transformer()
      
      '// Capture a number followed by 'in' and convert it.
      '// Note: `len` is a string, so we must use Double(len) for the calculation.
      t.FromTo("{@Number:len}in", "{@Eval: Double(len) * 2.54}cm")
      
      Console.WriteLine(t.Transform("The board is 10in long."))
   End Sub
End Module
				
			
The board is 25.4cm long.
A practical example that calculates line-item totals for a list of products by capturing quantity and price.
				
					using uCalcSoftware;

var uc = new uCalc();
var t = new uCalc.Transformer();
// Rule to find quantity and price, then calculate total.
// Note the explicit Double() conversion for the captured text variables.
t.FromTo("Qty: {@Number:qty}, Price: {@Number:price}",
"{@Self}, Total: {@Eval: Double(qty) * Double(price)}");

var invoice = """
Item: Book, Qty: 3, Price: 15.00
Item: Pen, Qty: 10, Price: 1.50
""";

Console.WriteLine(t.Transform(invoice));
				
			
Item: Book, Qty: 3, Price: 15.00, Total: 45
Item: Pen, Qty: 10, Price: 1.50, Total: 15
				
					#include <iostream>
#include "uCalc.h"

using namespace std;
using namespace uCalcSoftware;

int main() {
   uCalc uc;
   uCalc::Transformer t;
   // Rule to find quantity and price, then calculate total.
   // Note the explicit Double() conversion for the captured text variables.
   t.FromTo("Qty: {@Number:qty}, Price: {@Number:price}",
   "{@Self}, Total: {@Eval: Double(qty) * Double(price)}");

   auto invoice = R"(Item: Book, Qty: 3, Price: 15.00
Item: Pen, Qty: 10, Price: 1.50)";

   cout << t.Transform(invoice) << endl;
}
				
			
Item: Book, Qty: 3, Price: 15.00, Total: 45
Item: Pen, Qty: 10, Price: 1.50, Total: 15
				
					Imports System
Imports uCalcSoftware
Public Module Program
   Public Sub Main()
      Dim uc As New uCalc()
      Dim t As New uCalc.Transformer()
      '// Rule to find quantity and price, then calculate total.
      '// Note the explicit Double() conversion for the captured text variables.
      t.FromTo("Qty: {@Number:qty}, Price: {@Number:price}",
      "{@Self}, Total: {@Eval: Double(qty) * Double(price)}")
      
      Dim invoice = "Item: Book, Qty: 3, Price: 15.00
Item: Pen, Qty: 10, Price: 1.50"
      
      Console.WriteLine(t.Transform(invoice))
   End Sub
End Module
				
			
Item: Book, Qty: 3, Price: 15.00, Total: 45
Item: Pen, Qty: 10, Price: 1.50, Total: 15
Internal Test: Verifies that `{@@Eval}` can correctly parse and evaluate an expression that is itself captured from the text.
				
					using uCalcSoftware;

var uc = new uCalc();
var t = new uCalc.Transformer();

// The variable {formula} will capture the literal text "10 * (5-2)".
// {@@Eval} then executes that text as an expression.
t.FromTo("solve({formula})", "'{formula}' is {@@Eval: formula}.");

var text = "The answer to solve(10 * (5-2))";
Console.WriteLine(t.Transform(text));
				
			
The answer to '10 * (5-2)' is 30.
				
					#include <iostream>
#include "uCalc.h"

using namespace std;
using namespace uCalcSoftware;

int main() {
   uCalc uc;
   uCalc::Transformer t;

   // The variable {formula} will capture the literal text "10 * (5-2)".
   // {@@Eval} then executes that text as an expression.
   t.FromTo("solve({formula})", "'{formula}' is {@@Eval: formula}.");

   auto text = "The answer to solve(10 * (5-2))";
   cout << t.Transform(text) << endl;
}
				
			
The answer to '10 * (5-2)' is 30.
				
					Imports System
Imports uCalcSoftware
Public Module Program
   Public Sub Main()
      Dim uc As New uCalc()
      Dim t As New uCalc.Transformer()
      
      '// The variable {formula} will capture the literal text "10 * (5-2)".
      '// {@@Eval} then executes that text as an expression.
      t.FromTo("solve({formula})", "'{formula}' is {@@Eval: formula}.")
      
      Dim text = "The answer to solve(10 * (5-2))"
      Console.WriteLine(t.Transform(text))
   End Sub
End Module
				
			
The answer to '10 * (5-2)' is 30.
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