You are given a long string and looking for certain patterns in the string.
The string contains only lowercase letters $(a-z)$, and it is represented in a compressed format. Denoting $S_1, S_2, ...$ as compressed strings, another compressed string $S$ is defined recursively in one of the following ways:
$\cdot$ $S$ can be any string consisting of only lowercase letters $(a-z)$.
$\cdot$ $S$ can be generated by repeating another string for any times. Specifically, $S$ is represented as “R(S1)”, which means that the content of $S_1$ is repeated $R$ times.
$\cdot$ $S$ can also be the concatenation of other strings. Specifically, $S$ is represented as “$S_1,S_2...S_L$”, which means $S$ is the concatenation of $S_1, S_2, ..., S_L$.
$\cdot$An empty string (“”) is also a valid representation.
Formally, the Backus–Naur Form (BNF) specification of the syntax is
<compressed> ::= “” | <lowercase-letter> | <compressed> <compressed> | <number> “(” <compressed> “)”
For example, the string “baaabbaaab” can be compressed as “b3(a)2(b)3(a)b”. It can also be compressed as “2(b3(a)b)”.
On the other hand, you find deterministic finite automaton (DFA) as powerful way to describe the patterns you are looking for. A DFA contains a finite set of states $Q$ and a finite set of input symbols called the alphabet Σ. Initially, the DFA is positioned at the start state $q_0∈Q$. Given the transition function $δ(q,a)$ and an input symbol $a$, the DFA transit to state $δ(q,a)$ if its current state is $q$.
Let $w=a_1 a_2...a_n$ be a string over the alphabet Σ. According to the above definition, the DFA transits through the following sequence of states.
$$q_0,q_1=δ(q_0,a_1 ),q_2=δ(q_1,a_2 ),…,q_n=δ(q_(n-1),a_n )$$
The DFA also contains a set of accept states $F\subseteq Q$. If the last state $q_n$ is an accept state, we say that the DFA accepts the string $w$. The set of accepted strings is referred as the language that the DFA represents.
Now you are given a compressed string $S$ and a DFA $A$. You want to know if $A$ accepts the decompressed content of $S$.