There are a number of important interactions in the Wolfram Language between evaluation and pattern matching. The first observation is that pattern matching is usually done on expressions that have already been at least partly evaluated. As a result, it is usually appropriate that the patterns to which these expressions are matched should themselves be evaluated.
There are some cases, however, where you may want to keep all or part of a pattern unevaluated. You can do this by wrapping the parts you do not want to evaluate with HoldPattern. In general, whenever HoldPattern[patt] appears within a pattern, this form is taken to be equivalent to patt for the purpose of pattern matching, but the expression patt is maintained unevaluated.
|HoldPattern[patt]||equivalent to patt for pattern matching, with patt kept unevaluated|
One application for HoldPattern is in specifying patterns which can apply to unevaluated expressions, or expressions held in an unevaluated form.
Notice that while functions like Hold prevent evaluation of expressions, they do not affect the manipulation of parts of those expressions with /. and other operators.
As illustrated above, the left‐hand sides of transformation rules such as lhs->rhs are usually evaluated immediately, since the rules are usually applied to expressions which have already been evaluated. The right‐hand side of lhs->rhs is also evaluated immediately. With the delayed rule lhs:>rhs, however, the expression rhs is not evaluated.
While the left‐hand sides of transformation rules are usually evaluated, the left‐hand sides of definitions are usually not. The reason for the difference is as follows. Transformation rules are typically applied using /. to expressions that have already been evaluated. Definitions, however, are used during the evaluation of expressions, and are applied to expressions that have not yet been completely evaluated. To work on such expressions, the left‐hand sides of definitions must be maintained in a form that is at least partially unevaluated.
Definitions for symbols are the simplest case. As discussed in "Non‐Standard Evaluation", a symbol on the left‐hand side of a definition such as x=value is not evaluated. If x had previously been assigned a value y, then if the left‐hand side of x=value were evaluated, it would turn into the quite unrelated definition y=value.
Although individual symbols that appear on the left‐hand sides of definitions are not evaluated, more complicated expressions are partially evaluated. In an expression such as f[args] on the left‐hand side of a definition, the args are evaluated.
You can see why the arguments of a function that appears on the left‐hand side of a definition must be evaluated by considering how the definition is used during the evaluation of an expression. As discussed in "Principles of Evaluation", when the Wolfram Language evaluates a function, it first evaluates each of the arguments, then tries to find definitions for the function. As a result, by the time the Wolfram Language applies any definition you have given for a function, the arguments of the function must already have been evaluated. An exception to this occurs when the function in question has attributes which specify that it should hold some of its arguments unevaluated.
|symbol=value||symbol is not evaluated; value is evaluated|
|symbol:=value||neither symbol nor value is evaluated|
|f[args]=value||args are evaluated; left‐hand side as a whole is not|
|f[HoldPattern[arg]]=value||f[arg] is assigned, without evaluating arg|
|Evaluate[lhs]=value||left‐hand side is evaluated completely|
While in most cases it is appropriate for the arguments of a function that appears on the left‐hand side of a definition to be evaluated, there are some situations in which you do not want this to happen. In such cases, you can wrap HoldPattern around the parts that you do not want to be evaluated.