String substitution

In computer science, in the area of formal language theory, frequent use is made of a variety of string functions; however, the notation used is different from that used for computer programming, and some commonly used functions in the theoretical realm are rarely used when programming. This article defines some of these basic terms.

A string is a finite sequence of characters. The empty string is denoted by ${\displaystyle \varepsilon }$. The concatenation of two string ${\displaystyle s}$ and ${\displaystyle t}$ is denoted by ${\displaystyle s\cdot t}$, or shorter by ${\displaystyle st}$. Concatenating with the empty string makes no difference: ${\displaystyle s\cdot \varepsilon =s=\varepsilon \cdot s}$. Concatenation of strings is associative: ${\displaystyle s\cdot (t\cdot u)=(s\cdot t)\cdot u}$.

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