Built-in Mathematica Symbol

Replace

Replace[expr, rules]
applies a rule or list of rules in an attempt to transform the entire expression expr.

Replace[expr, rules, levelspec]
applies rules to parts of expr specified by levelspec.

A list of rules can be given. The rules are tried in order. The result of the first one that applies is returned. If none of the rules apply, the original expr is returned.

If the rules are given in nested lists, Replace is effectively mapped onto the inner lists. Thus Replace[expr, {{r₁₁, r₁₂}, {r₂₁, ...}, ...}] is equivalent to {Replace[expr, {r₁₁, r₁₂}], Replace[expr, {r₂₁, ...}], ...}.

The default value for levelspec in Replace is {0}, corresponding to the whole expression.

Level -1 consists of numbers, symbols and other objects that do not have subparts.

With the option setting Heads->True, Replace includes heads of expressions, and their parts.

Replacements are performed to parts specified by levelspec even when those parts have Hold or related wrappers.

Replace by default applies rules only to complete expressions:

A list of rules gives a list of results: