Function Repository Resource:

ToExpressionMatched

Source Notebook

Interpret input and check that the resulting expression matches a specific pattern

Contributed by: Sjoerd Smit

ResourceFunction["ToExpressionMatched"][input,patt]

interprets input as an expression only if the unevaluated form matches patt.

ResourceFunction["ToExpressionMatched"][input,patt,form]

uses interpretation rules corresponding to the specified form.

ResourceFunction["ToExpressionMatched"][input,patt,form,h]

wraps the head h around the expression produced before evaluating it.

ResourceFunction["ToExpressionMatched"][input,patt,Automatic,h]

uses the default interpretation rules from ToExpression.

Details

ResourceFunction["ToExpressionMatched"] returns a Failure object if the interpretation of the input does not match the requested pattern.

ResourceFunction["ToExpressionMatched"] uses ToExpression[input,form,HoldComplete] to parse the input, and then checks that it returns something that matches HoldComplete[patt] before removing HoldComplete or replacing it with a new head h.

ResourceFunction["ToExpressionMatched"][input,patt,Automatic,h] will use InputForm for string inputs and StandardForm otherwise.

Examples

Basic Examples (3)

Interpret a string, expecting an integer:

In[1]:=

Out[1]=

A failure is generated if the result does not match:

In[2]:=

Out[2]=

The pattern is checked before evaluation, so this fails because the head of the interpreted input is Plus:

In[3]:=

Out[3]=

Check that the head is Plus:

In[4]:=

Out[4]=

Instead, check that the input is a sum of integers:

In[5]:=

Out[5]=

If the input cannot be interpreted as a valid expression, ToExpressionMatched behaves similarly as ToExpression, but it generates a Failure object instead of $Failed:

In[6]:=