How do I work with Regex for Regular Expressions in Rust?

Walkthrough

Regex is a crate for parsing, compiling, and executing regular expressions in Rust. It provides a mature, well-tested implementation with a familiar syntax similar to other regex engines. The crate offers both simple pattern matching and advanced features like capture groups, lookahead, and replacement with callbacks.

Key concepts:

Pattern compilation — Compile regex patterns with Regex::new()
Matching — Test if text matches with is_match()
Finding — Locate matches with find() and find_iter()
Capture groups — Extract matched groups with captures()
Replacement — Replace matches with replace()
Flags — Control behavior with inline flags like (?i)

When to use Regex:

Validating input formats (email, phone numbers, etc.)
Parsing structured text
Search and replace operations
Log file analysis
Data extraction from text

When NOT to use Regex:

Simple string operations (use str::contains, str::starts_with, etc.)
Parsing nested structures (use a parser combinator like nom)
Very performance-critical inner loops
Complex grammar parsing (use a proper parser)

Code Examples

Basic Pattern Matching

use regex::Regex;
 
fn main() {
    let re = Regex::new(r"hello").unwrap();
    
    assert!(re.is_match("hello world"));
    assert!(!re.is_match("goodbye"));
    
    println!("Pattern matches!");
}

Finding Matches

use regex::Regex;
 
fn main() {
    let re = Regex::new(r"\d+").unwrap();
    let text = "There are 42 cats and 7 dogs.";
    
    // Find all matches
    for cap in re.find_iter(text) {
        println!("Found: {}", cap.as_str());
    }
    
    // Check if pattern exists
    if let Some(m) = re.find(text) {
        println!("First match: {}", m.as_str());
    }
}

Capture Groups

use regex::Regex;
 
fn main() {
    let re = Regex::new(r"(\d+)-(\d+)-(\d+)").unwrap();
    let text = "Date: 2024-01-15";
    
    if let Some(caps) = re.captures(text) {
        println!("Full match: {}", &caps[0]);
        println!("Year: {}", &caps[1]);
        println!("Month: {}", &caps[2]);
        println!("Day: {}", &caps[3]);
    }
}

Named Capture Groups

use regex::Regex;
 
fn main() {
    let re = Regex::new(r"(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})").unwrap();
    let text = "Date: 2024-01-15";
    
    if let Some(caps) = re.captures(text) {
        println!("Year: {}", &caps["year"]);
        println!("Month: {}", &caps["month"]);
        println!("Day: {}", &caps["day"]);
    }
}

Simple Replacement

use regex::Regex;
 
fn main() {
    let re = Regex::new(r"\d+").unwrap();
    let text = "There are 42 cats and 7 dogs.";
    
    // Replace with string
    let result = re.replace(text, "X");
    println!("{}", result);  // "There are X cats and X dogs."
    
    // Replace first occurrence only
    let result = re.replace(text, "X");
    println!("First only: {}", result);
}

Replacement with Callback

use regex::Regex;
 
fn main() {
    let re = Regex::new(r"\d+").unwrap();
    let text = "Ages: 25, 30, 45";
    
    // Replace with a function
    let result = re.replace_all(text, |caps: &regex::Captures| {
        let num: i32 = caps[0].parse().unwrap();
        (num + 1).to_string()
    });
    
    println!("{}", result);  // "Ages: 26, 31, 46"
}

Replacement with Capture References

use regex::Regex;
 
fn main() {
    // Swap first and last name
    let re = Regex::new(r"(\w+) (\w+)").unwrap();
    let text = "John Smith, Jane Doe";
    
    let result = re.replace_all(text, "$2 $1");
    println!("{}", result);  // "Smith John, Doe Jane"
}

Case-Insensitive Matching

use regex::Regex;
 
fn main() {
    // Inline flag for case insensitivity
    let re = Regex::new(r"(?i)hello").unwrap();
    
    assert!(re.is_match("Hello"));
    assert!(re.is_match("HELLO"));
    assert!(re.is_match("hello"));
    
    // Or use the builder
    let re = Regex::builder()
        .case_insensitive(true)
        .build(r"hello")
        .unwrap();
    
    assert!(re.is_match("HELLO"));
}

Anchors and Boundaries

use regex::Regex;
 
fn main() {
    let text = "the cat and the dog";
    
    // Start of string
    let start = Regex::new(r"^the").unwrap();
    assert!(start.is_match(text));
    
    // End of string
    let end = Regex::new(r"dog$").unwrap();
    assert!(end.is_match(text));
    
    // Word boundary
    let word = Regex::new(r"\bcat\b").unwrap();
    assert!(word.is_match(text));
    
    // Not a word boundary
    let not_word = Regex::new(r"\Bcat\B").unwrap();
    assert!(!not_word.is_match("category"));
}

Quantifiers

use regex::Regex;
 
fn main() {
    let text = "aaa b aaaa";
    
    // Zero or more (greedy)
    let re = Regex::new(r"a*").unwrap();
    
    // One or more
    let re = Regex::new(r"a+").unwrap();
    
    // Zero or one
    let re = Regex::new(r"a?").unwrap();
    
    // Exact count
    let re = Regex::new(r"a{3}").unwrap();  // Exactly 3
    assert!(re.is_match("aaa"));
    
    // Range
    let re = Regex::new(r"a{2,4}").unwrap();  // 2 to 4
    assert!(re.is_match("aaaa"));
    
    // Non-greedy (minimal match)
    let re = Regex::new(r"a+?").unwrap();
}

Character Classes

use regex::Regex;
 
fn main() {
    // Digit
    let digit = Regex::new(r"\d").unwrap();
    assert!(digit.is_match("5"));
    
    // Non-digit
    let non_digit = Regex::new(r"\D").unwrap();
    assert!(non_digit.is_match("a"));
    
    // Word character
    let word = Regex::new(r"\w").unwrap();
    assert!(word.is_match("a"));
    assert!(word.is_match("_"));
    
    // Whitespace
    let space = Regex::new(r"\s").unwrap();
    assert!(space.is_match(" "));
    
    // Custom character class
    let vowels = Regex::new(r"[aeiou]").unwrap();
    assert!(vowels.is_match("hello"));
    
    // Negated character class
    let non_vowels = Regex::new(r"[^aeiou]").unwrap();
    assert!(non_vowels.is_match("h"));
    
    // Range in character class
    let lowercase = Regex::new(r"[a-z]").unwrap();
}

Alternation

use regex::Regex;
 
fn main() {
    let re = Regex::new(r"cat|dog|bird").unwrap();
    
    assert!(re.is_match("I have a cat"));
    assert!(re.is_match("I have a dog"));
    assert!(re.is_match("I have a bird"));
    assert!(!re.is_match("I have a fish"));
}

Email Validation

use regex::Regex;
 
fn is_valid_email(email: &str) -> bool {
    let re = Regex::new(
        r"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$"
    ).unwrap();
    re.is_match(email)
}
 
fn main() {
    assert!(is_valid_email("user@example.com"));
    assert!(is_valid_email("user.name+tag@example.co.uk"));
    assert!(!is_valid_email("invalid-email"));
    assert!(!is_valid_email("@example.com"));
    
    println!("Email validation works!");
}

Phone Number Extraction

use regex::Regex;
 
fn extract_phone_numbers(text: &str) -> Vec<String> {
    let re = Regex::new(
        r"\b\d{3}[-.]?\d{3}[-.]?\d{4}\b"
    ).unwrap();
    
    re.find_iter(text)
        .map(|m| m.as_str().to_string())
        .collect()
}
 
fn main() {
    let text = "Call 555-123-4567 or 555.987.6543 or 5551112222";
    let numbers = extract_phone_numbers(text);
    
    for num in &numbers {
        println!("Found: {}", num);
    }
}

URL Parsing

use regex::Regex;
 
#[derive(Debug)]
struct Url {
    scheme: String,
    domain: String,
    path: Option<String>,
}
 
fn parse_url(url: &str) -> Option<Url> {
    let re = Regex::new(
        r"^(?P<scheme>https?)://(?P<domain>[^/]+)(?P<path>/.*)?$"
    ).unwrap();
    
    re.captures(url).map(|caps| Url {
        scheme: caps["scheme"].to_string(),
        domain: caps["domain"].to_string(),
        path: caps.name("path").map(|m| m.as_str().to_string()),
    })
}
 
fn main() {
    let urls = vec![
        "https://example.com",
        "http://example.com/path/to/page",
    ];
    
    for url_str in urls {
        if let Some(url) = parse_url(url_str) {
            println!("{:#?}", url);
        }
    }
}

Log Parsing

use regex::Regex;
 
#[derive(Debug)]
struct LogEntry {
    timestamp: String,
    level: String,
    message: String,
}
 
fn parse_log_line(line: &str) -> Option<LogEntry> {
    let re = Regex::new(
        r"\[(?P<timestamp>[^\]]+)\]\s+(?P<level>\w+):\s+(?P<message>.+)"
    ).unwrap();
    
    re.captures(line).map(|caps| LogEntry {
        timestamp: caps["timestamp"].to_string(),
        level: caps["level"].to_string(),
        message: caps["message"].to_string(),
    })
}
 
fn main() {
    let log = "[2024-01-15 10:30:00] ERROR: Connection failed";
    
    if let Some(entry) = parse_log_line(log) {
        println!("{:#?}", entry);
    }
}

Split on Regex

use regex::Regex;
 
fn main() {
    let re = Regex::new(r"[,\s]+").unwrap();
    let text = "apple, banana,  cherry, orange";
    
    // Split on pattern
    let parts: Vec<&str> = re.split(text).collect();
    println!("{:?}", parts);  // ["apple", "banana", "cherry", "orange"]
    
    // Limited split
    let parts: Vec<&str> = re.splitn(text, 2).collect();
    println!("{:?}", parts);  // ["apple", "banana,  cherry, orange"]
}

Matching Multiple Lines

use regex::Regex;
 
fn main() {
    let text = "first line\nsecond line\nthird line";
    
    // Multi-line mode (^ and $ match line boundaries)
    let re = Regex::new(r"(?m)^second").unwrap();
    assert!(re.is_match(text));
    
    // Match newline explicitly
    let re = Regex::new(r"first\nsecond").unwrap();
    assert!(re.is_match(text));
}

Dot Matches Newline

use regex::Regex;
 
fn main() {
    let text = "hello\nworld";
    
    // By default, dot doesn't match newline
    let re = Regex::new(r"hello.world").unwrap();
    assert!(!re.is_match(text));
    
    // With s flag, dot matches everything including newline
    let re = Regex::new(r"(?s)hello.world").unwrap();
    assert!(re.is_match(text));
}

Escape Metacharacters

use regex::Regex;
 
fn main() {
    // Escape special characters for literal matching
    let pattern = regex::escape("(a+b)*c");
    // pattern is now "\\(a\\+b\\)\\*c"
    
    let re = Regex::new(&pattern).unwrap();
    assert!(re.is_match("(a+b)*c"));
}

Compile Once with Lazy Static

use regex::Regex;
use std::sync::LazyLock;
 
// Compile regex once, reuse everywhere
static EMAIL_REGEX: LazyLock<Regex> = LazyLock::new(|| {
    Regex::new(r"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$").unwrap()
});
 
fn is_valid_email(email: &str) -> bool {
    EMAIL_REGEX.is_match(email)
}
 
fn main() {
    println!("Valid: {}", is_valid_email("user@example.com"));
}

RegexSet for Multiple Patterns

use regex::RegexSet;
 
fn main() {
    let set = RegexSet::new(&[
        r"\w+",
        r"\d+",
        r"\pL+",
        r"foo",
        r"bar",
    ]).unwrap();
    
    let matches: Vec<_> = set.matches("bar").into_iter().collect();
    println!("Matches: {:?}", matches);  // [0, 2, 4]
    
    // Check if any match
    assert!(set.is_match("bar"));
}

Performance Tips

use regex::Regex;
 
fn main() {
    // Prefer non-capturing groups when you don't need captures
    let re = Regex::new(r"(?:https?|ftp)://\S+").unwrap();
    
    // Use atomic groups for performance on large inputs
    // let re = Regex::new(r"(?>a|b|c)").unwrap();
    
    // Compile regex outside loops
    let re = Regex::new(r"\d+").unwrap();
    
    let texts = vec!["a1", "b2", "c3"];
    for text in texts {
        // Reuse the compiled regex
        if re.is_match(text) {
            println!("Match in: {}", text);
        }
    }
}

Testing Regex Patterns

use regex::Regex;
 
#[cfg(test)]
mod tests {
    use super::*;
    
    #[test]
    fn test_phone_pattern() {
        let re = Regex::new(r"\b\d{3}[-.]?\d{3}[-.]?\d{4}\b").unwrap();
        
        assert!(re.is_match("555-123-4567"));
        assert!(re.is_match("555.123.4567"));
        assert!(re.is_match("5551234567"));
        assert!(!re.is_match("55-123-4567"));
    }
    
    #[test]
    fn test_capture_groups() {
        let re = Regex::new(r"(\d{4})-(\d{2})-(\d{2})").unwrap();
        let caps = re.captures("2024-01-15").unwrap();
        
        assert_eq!(&caps[1], "2024");
        assert_eq!(&caps[2], "01");
        assert_eq!(&caps[3], "15");
    }
}
 
fn main() {
    println!("Tests defined");
}

Summary

Regex Key Imports:

use regex::Regex;
use regex::RegexSet;

Core Methods:

| Method | Description | |--------|-------------| | Regex::new(pattern) | Compile pattern | | is_match(text) | Check if pattern matches | | find(text) | Find first match | | find_iter(text) | Iterate all matches | | captures(text) | Get capture groups | | replace(text, replacement) | Replace matches | | split(text) | Split on pattern |

Common Patterns:

| Pattern | Description | |---------|-------------| | \d | Digit | | \w | Word character | | \s | Whitespace | | \b | Word boundary | | ^ | Start of string | | $ | End of string | | . | Any character | | * | Zero or more | | + | One or more | | ? | Zero or one |

Flags:

| Flag | Description | |------|-------------| | (?i) | Case insensitive | | (?m) | Multi-line mode | | (?s) | Dot matches newline | | (?x) | Ignore whitespace |

Escape Metacharacters:

let escaped = regex::escape("(a+b)*c");

Key Points:

Compile regex outside loops for performance
Use LazyLock or once_cell for global patterns
Use find_iter() for all matches
Use captures() for extracting groups
Named groups with (?P<name>...) improve readability
Use RegexSet when matching multiple patterns
Escape user input with regex::escape()
Prefer simple string methods when possible

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