String interpolation

In computer programming, string interpolation (or variable interpolation, variable substitution, or variable expansion) is the process of evaluating a string literal containing one or more placeholders, yielding a result in which the placeholders are replaced with their corresponding values. It is a form of simple template processing or, in formal terms, a form of quasi-quotation (or logic substitution interpretation). String interpolation allows easier and more intuitive string formatting and content-specification compared with string concatenation.

String interpolation is common in many programming languages which make heavy use of string representations of data, such as Groovy, Kotlin, Perl, PHP, Python, Ruby, Scala, and Swift, and most Unix shells. Two modes of literal expression are usually offered: one with interpolation enabled, the other without (termed raw string). Placeholders are usually represented by a bare or a named sigil (typically $ or %), e.g. $placeholder or %123. Expansion of the string usually occurs at run time.

Some languages do not offer string interpolation, instead offering a standard function where one parameter is the printf format string, and other(s) provide the values for each placeholder.

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