rust 33 lines · 7 steps

Parsing quoted fields with a state machine in Rust

A single-pass scanner that walks a string character by character, tracking quote and escape state to pull out quoted substrings.

Explained by highlit
Intermediate state-machine parsing string-processing escaping iterators
1pub fn extract_quoted_fields(line: &str) -> Vec<String> {
2 let mut fields = Vec::new();
3 let mut current = String::new();
4 let mut in_quotes = false;
5 let mut escaped = false;
6 let mut chars = line.chars();
7 
8 while let Some(c) = chars.next() {
9 if escaped {
10 match c {
11 'n' => current.push('\n'),
12 't' => current.push('\t'),
13 'r' => current.push('\r'),
14 other => current.push(other),
15 }
16 escaped = false;
17 continue;
18 }
19 
20 match c {
21 '\\' if in_quotes => escaped = true,
22 '"' if in_quotes => {
23 fields.push(std::mem::take(&mut current));
24 in_quotes = false;
25 }
26 '"' => in_quotes = true,
27 _ if in_quotes => current.push(c),
28 _ => {}
29 }
30 }
31 
32 fields
33}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A few boolean flags turn a character iterator into a tiny state machine that parses without regex or backtracking.
  2. 2std::mem::take swaps a value out and leaves a default behind, avoiding a clone when you want to keep reusing the buffer.
  3. 3Handling the escape case before the main match keeps escaped characters from being interpreted as syntax.

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