Regex

Regular expressions (regex) are powerful tools for pattern matching and text manipulation. Here's how different languages handle regex operations.

Basic Pattern Matching

JavaScript

// Create regex
const pattern = /hello/i;  // literal with flag
const pattern2 = new RegExp("hello", "i");  // constructor

// Test match
pattern.test("Hello World");  // true

// Find match
"Hello World".match(/hello/i);  // ["Hello"]
"Hello World".search(/hello/i);  // 0 (index)

// Find all matches
"hello hello".match(/hello/g);  // ["hello", "hello"]

PHP

<?php
// Create regex
$pattern = "/hello/i";

// Test match
preg_match($pattern, "Hello World");  // 1 (true)

// Find match
preg_match($pattern, "Hello World", $matches);  // $matches[0] = "Hello"

// Find all matches
preg_match_all("/hello/i", "hello hello", $matches);  // $matches[0] = ["hello", "hello"]

Rust

use regex::Regex;

// Create regex
let re = Regex::new(r"hello")?;
let re_case = Regex::new(r"(?i)hello")?;  // case insensitive

// Test match
re.is_match("Hello World");  // false (case sensitive)
re_case.is_match("Hello World");  // true

// Find match
re.find("Hello World");  // Some(Match)
if let Some(mat) = re.find("Hello World") {
  println!("{}", mat.as_str());  // "hello"
}

// Find all matches
for mat in re.find_iter("hello hello") {
  println!("{}", mat.as_str());
}

Go

import (
  "regexp"
)

// Create regex
re, _ := regexp.Compile("hello")
reCase, _ := regexp.Compile("(?i)hello")  // case insensitive

// Test match
re.MatchString("Hello World")  // false
reCase.MatchString("Hello World")  // true

// Find match
re.FindString("Hello World")  // "hello" or ""

// Find all matches
re.FindAllString("hello hello", -1)  // ["hello", "hello"]

// Find with index
re.FindStringIndex("Hello World")  // [0, 5]

Python

import re

# Create regex
pattern = re.compile(r"hello", re.IGNORECASE)

# Test match
pattern.search("Hello World")  # Match object or None
bool(pattern.search("Hello World"))  # True

# Find match
pattern.search("Hello World").group()  # "Hello"
pattern.findall("hello hello")  # ["hello", "hello"]

# Find with index
match = pattern.search("Hello World")
if match:
  match.start()  # 0
  match.end()    # 5
  match.span()   # (0, 5)

Zig

// Zig doesn't have built-in regex
// Use external library like zig-regex or pcre

// Example with external library
const std = @import("std");
// const regex = @import("regex");

// Manual pattern matching for simple cases
const str = "Hello World";
const pattern = "hello";
// Use std.mem.indexOfPos for simple cases

C#

using System.Text.RegularExpressions;

// Create regex
Regex pattern = new Regex("hello", RegexOptions.IgnoreCase);

// Test match
pattern.IsMatch("Hello World");  // true

// Find match
Match match = pattern.Match("Hello World");
if (match.Success) {
  string result = match.Value;  // "Hello"
}

// Find all matches
MatchCollection matches = pattern.Matches("hello hello");
foreach (Match m in matches) {
  Console.WriteLine(m.Value);
}

C++

#include <regex>
#include <string>

// Create regex
std::regex pattern("hello", std::regex_constants::icase);

// Test match
std::regex_search("Hello World", pattern);  // true

// Find match
std::smatch match;
if (std::regex_search("Hello World", match, pattern)) {
  std::string result = match.str();  // "Hello"
}

// Find all matches
std::string text = "hello hello";
std::sregex_iterator iter(text.begin(), text.end(), pattern);
std::sregex_iterator end;
for (; iter != end; ++iter) {
  std::cout << iter->str() << std::endl;
}

C

// C doesn't have built-in regex
// Use POSIX regex (regex.h) or PCRE library

#include <regex.h>

regex_t regex;
int reti = regcomp(&regex, "hello", REG_ICASE | REG_EXTENDED);

if (reti) {
  // error
}

// Test match
reti = regexec(&regex, "Hello World", 0, NULL, 0);
if (!reti) {
  // match found
}

regfree(&regex);  // free

Java

import java.util.regex.Pattern;
import java.util.regex.Matcher;

// Create regex
Pattern pattern = Pattern.compile("hello", Pattern.CASE_INSENSITIVE);

// Test match
pattern.matcher("Hello World").find();  // true

// Find match
Matcher matcher = pattern.matcher("Hello World");
if (matcher.find()) {
  String result = matcher.group();  // "Hello"
}

// Find all matches
Matcher matcher = pattern.matcher("hello hello");
while (matcher.find()) {
  System.out.println(matcher.group());
}

Ruby

# Create regex
pattern = /hello/i

# Test match
pattern =~ "Hello World"  # 0 (index) or nil
"Hello World" =~ pattern  # 0

# Find match
"Hello World".match(pattern)  # MatchData object
"Hello World"[pattern]  # "Hello"

# Find all matches
"hello hello".scan(pattern)  # ["hello", "hello"]

# Match with groups
/(\w+) (\w+)/.match("Hello World")
# $1 = "Hello", $2 = "World"

Swift

import Foundation

// Create regex
let pattern = try NSRegularExpression(pattern: "hello", options: .caseInsensitive)

// Test match
let range = NSRange(location: 0, length: str.utf16.count)
pattern.numberOfMatches(in: str, range: range) > 0  // true

// Find match
if let match = pattern.firstMatch(in: str, range: range) {
  let result = (str as NSString).substring(with: match.range)
}

// Find all matches
let matches = pattern.matches(in: str, range: range)
for match in matches {
  let result = (str as NSString).substring(with: match.range)
}

Replace Operations

JavaScript

// Replace first match
"Hello World".replace(/hello/i, "Hi");  // "Hi World"

// Replace all matches
"hello hello".replace(/hello/g, "Hi");  // "Hi Hi"

// Replace with function
"hello world".replace(/\w+/g, (match) => match.toUpperCase());  // "HELLO WORLD"

PHP

<?php
// Replace first match
preg_replace("/hello/i", "Hi", "Hello World");  // "Hi World"

// Replace all matches (default behavior)
preg_replace("/hello/i", "Hi", "hello hello");  // "Hi Hi"

// Replace with callback
preg_replace_callback("/\w+/", function($matches) {
  return strtoupper($matches[0]);
}, "hello world");  // "HELLO WORLD"

Rust

use regex::Regex;

let re = Regex::new(r"hello")?;

// Replace all matches
re.replace_all("hello hello", "Hi");  // "Hi Hi"

// Replace with function
re.replace_all("hello world", |caps: &regex::Captures| {
  caps[0].to_uppercase()
});  // "HELLO WORLD"

Go

import "regexp"

re, _ := regexp.Compile("hello")

// Replace all matches
re.ReplaceAllString("hello hello", "Hi")  // "Hi Hi"

// Replace with function
re.ReplaceAllStringFunc("hello world", func(s string) string {
  return strings.ToUpper(s)
})  // "HELLO WORLD"

Python

import re

pattern = re.compile(r"hello")

# Replace all matches
pattern.sub("Hi", "hello hello")  # "Hi Hi"
re.sub(r"hello", "Hi", "hello hello")  # "Hi Hi"

# Replace with function
pattern.sub(lambda m: m.group().upper(), "hello world")  # "HELLO WORLD"

# Replace with count
pattern.sub("Hi", "hello hello", count=1)  # "Hi hello"

Zig

// Use external regex library
// Manual replacement for simple cases
// const std = @import("std");
// Use std.mem.replace for simple string replacement

C#

using System.Text.RegularExpressions;

Regex pattern = new Regex("hello");

// Replace all matches
pattern.Replace("hello hello", "Hi");  // "Hi Hi"

// Replace with function
pattern.Replace("hello world", match => {
  return match.Value.ToUpper();
});  // "HELLO WORLD"

C++

#include <regex>

std::regex pattern("hello");

// Replace all matches
std::string result = std::regex_replace(
  "hello hello", 
  pattern, 
  "Hi"
);  // "Hi Hi"

// Replace with format
std::regex_replace("hello world", pattern, 
  [](const std::smatch& m) {
      std::string s = m.str();
      std::transform(s.begin(), s.end(), s.begin(), ::toupper);
      return s;
  }
);

C

// Use POSIX regex or PCRE library
// Manual implementation or use library functions
// regcomp, regexec, regfree for matching
// Manual string replacement for simple cases

Java

import java.util.regex.Pattern;

Pattern pattern = Pattern.compile("hello");

// Replace all matches
pattern.matcher("hello hello").replaceAll("Hi");  // "Hi Hi"

// Replace first match
pattern.matcher("hello hello").replaceFirst("Hi");  // "Hi hello"

// Replace with function (Java 9+)
pattern.matcher("hello world").replaceAll(mr -> 
  mr.group().toUpperCase()
);  // "HELLO WORLD"

Ruby

pattern = /hello/

# Replace all matches
"hello hello".gsub(pattern, "Hi")  # "Hi Hi"

# Replace first match
"hello hello".sub(pattern, "Hi")  # "Hi hello"

# Replace with block
"hello world".gsub(/\w+/) { |match| match.upcase }  # "HELLO WORLD"

Swift

import Foundation

let pattern = try NSRegularExpression(pattern: "hello")

// Replace all matches
let range = NSRange(location: 0, length: str.utf16.count)
let result = pattern.stringByReplacingMatches(
  in: str,
  range: range,
  withTemplate: "Hi"
)  // "Hi Hi"

// Replace with function (manual implementation)
var result = str
let matches = pattern.matches(in: str, range: range)
for match in matches.reversed() {
  let replacement = "Hi"
  result = (result as NSString).replacingCharacters(
      in: match.range, 
      with: replacement
  )
}

Capture Groups

JavaScript

// Capture groups
const match = "John Doe".match(/(\w+) (\w+)/);
// match[0] = "John Doe"
// match[1] = "John"
// match[2] = "Doe"

// Named groups (ES2018+)
const named = /(?<first>\w+) (?<last>\w+)/.exec("John Doe");
named.groups.first;  // "John"
named.groups.last;   // "Doe"

// Use in replace
"John Doe".replace(/(\w+) (\w+)/, "$2, $1");  // "Doe, John"

PHP

<?php
// Capture groups
preg_match("/(\w+) (\w+)/", "John Doe", $matches);
// $matches[0] = "John Doe"
// $matches[1] = "John"
// $matches[2] = "Doe"

// Named groups
preg_match("/(?<first>\w+) (?<last>\w+)/", "John Doe", $matches);
$matches['first'];  // "John"
$matches['last'];   // "Doe"

// Use in replace
preg_replace("/(\w+) (\w+)/", "$2, $1", "John Doe");  // "Doe, John"

Rust

use regex::Regex;

let re = Regex::new(r"(\w+) (\w+)")?;

if let Some(caps) = re.captures("John Doe") {
  let full = &caps[0];  // "John Doe"
  let first = &caps[1];  // "John"
  let last = &caps[2];   // "Doe"
}

// Named groups
let re = Regex::new(r"(?<first>\w+) (?<last>\w+)")?;
if let Some(caps) = re.captures("John Doe") {
  let first = &caps["first"];  // "John"
  let last = &caps["last"];    // "Doe"
}

// Use in replace
re.replace("John Doe", "$last, $first");  // "Doe, John"

Go

import "regexp"

re := regexp.MustCompile("(\w+) (\w+)")

// Capture groups
matches := re.FindStringSubmatch("John Doe")
// matches[0] = "John Doe"
// matches[1] = "John"
// matches[2] = "Doe"

// Named groups (Go 1.11+)
re = regexp.MustCompile("(?P<first>\w+) (?P<last>\w+)")
matches := re.FindStringSubmatch("John Doe")
names := re.SubexpNames()
// Access by index or use map

// Use in replace
re.ReplaceAllString("John Doe", "$2, $1")  // "Doe, John"

Python

import re

pattern = re.compile(r"(\w+) (\w+)")

# Capture groups
match = pattern.search("John Doe")
match.group(0)  # "John Doe"
match.group(1)  # "John"
match.group(2)  # "Doe"
match.groups()  # ("John", "Doe")

# Named groups
pattern = re.compile(r"(?P<first>\w+) (?P<last>\w+)")
match = pattern.search("John Doe")
match.group("first")  # "John"
match.group("last")   # "Doe"
match.groupdict()     # {"first": "John", "last": "Doe"}

# Use in replace
pattern.sub(r"\2, \1", "John Doe")  # "Doe, John"

Zig

// Use external regex library
// Capture groups supported by most regex libraries

C#

using System.Text.RegularExpressions;

Regex pattern = new Regex("(\w+) (\w+)");

Match match = pattern.Match("John Doe");
if (match.Success) {
  string full = match.Groups[0].Value;  // "John Doe"
  string first = match.Groups[1].Value;  // "John"
  string last = match.Groups[2].Value;  // "Doe"
}

// Named groups
Regex named = new Regex("(?<first>\w+) (?<last>\w+)");
Match m = named.Match("John Doe");
string first = m.Groups["first"].Value;  // "John"
string last = m.Groups["last"].Value;   // "Doe"

// Use in replace
pattern.Replace("John Doe", "$2, $1");  // "Doe, John"

C++

#include <regex>

std::regex pattern("(\w+) (\w+)");

std::smatch match;
if (std::regex_search("John Doe", match, pattern)) {
  std::string full = match[0];   // "John Doe"
  std::string first = match[1];   // "John"
  std::string last = match[2];   // "Doe"
}

// Use in replace
std::regex_replace("John Doe", pattern, "$2, $1");  // "Doe, John"

C

// Use POSIX regex with regmatch_t
#include <regex.h>

regex_t regex;
regmatch_t matches[3];  // 0=full, 1=first group, 2=second group

regcomp(&regex, "\(\w\+\) \(\w\+\)", REG_EXTENDED);
regexec(&regex, "John Doe", 3, matches, 0);

// Access matches[0].rm_so, matches[0].rm_eo for positions
// Extract substring manually

Java

import java.util.regex.Pattern;
import java.util.regex.Matcher;

Pattern pattern = Pattern.compile("(\w+) (\w+)");
Matcher matcher = pattern.matcher("John Doe");

if (matcher.find()) {
  String full = matcher.group(0);   // "John Doe"
  String first = matcher.group(1);  // "John"
  String last = matcher.group(2);   // "Doe"
}

// Named groups (Java 7+)
Pattern named = Pattern.compile("(?<first>\w+) (?<last>\w+)");
Matcher m = named.matcher("John Doe");
if (m.find()) {
  String first = m.group("first");  // "John"
  String last = m.group("last");    // "Doe"
}

// Use in replace
matcher.replaceAll("$2, $1");  // "Doe, John"

Ruby

pattern = /(\w+) (\w+)/

# Capture groups
match = pattern.match("John Doe")
match[0]  # "John Doe"
match[1]  # "John"
match[2]  # "Doe"

# Named groups
pattern = /(?<first>\w+) (?<last>\w+)/
match = pattern.match("John Doe")
match[:first]  # "John"
match["last"]  # "Doe"

# Use in replace
"John Doe".gsub(pattern, "\2, \1")  # "Doe, John"

Swift

import Foundation

let pattern = try NSRegularExpression(
  pattern: "(\w+) (\w+)", 
  options: []
)

let range = NSRange(location: 0, length: str.utf16.count)
if let match = pattern.firstMatch(in: str, range: range) {
  let full = (str as NSString).substring(with: match.range)
  if match.numberOfRanges > 1 {
      let first = (str as NSString).substring(with: match.range(at: 1))
  }
  if match.numberOfRanges > 2 {
      let last = (str as NSString).substring(with: match.range(at: 2))
  }
}

// Named groups (manual extraction or use external library)

Common Patterns

JavaScript

// Email
/^[\w\.-]+@[\w\.-]+\.[a-z]{2,}$/i

// Phone (US)
/^\+?1?[-. ]?\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$/

// URL
/https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*)/

// Digits only
/^\d+$/

// Alphanumeric
/^[a-zA-Z0-9]+$/

PHP

<?php
// Email
"/^[\w\.-]+@[\w\.-]+\.[a-z]{2,}$/i"

// Phone (US)
"/^\+?1?[-. ]?\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$/"

// URL
"/https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&\/\/=]*)/"

// Digits only
"/^\d+$/"

// Alphanumeric
"/^[a-zA-Z0-9]+$/"

Rust

use regex::Regex;

// Email
Regex::new(r"^[w.-]+@[w.-]+.[a-z]{2,}$")?;

// Phone (US)
Regex::new(r"^+?1?[-. ]?(?([0-9]{3}))?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$")?;

// URL
Regex::new(r"https?://(www.)?[-a-zA-Z0-9@:%._+~#=]{1,256}.[a-zA-Z0-9()]{1,6}([-a-zA-Z0-9()@:%_+.~#?&//=]*)")?;

// Digits only
Regex::new(r"^d+$")?;

// Alphanumeric
Regex::new(r"^[a-zA-Z0-9]+$")?;

Go

import "regexp"

// Email
regexp.MustCompile("^[\w\.-]+@[\w\.-]+\.[a-z]{2,}$")

// Phone (US)
regexp.MustCompile("^\+?1?[-. ]?\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$")

// URL
regexp.MustCompile("https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&\/\/=]*)")

// Digits only
regexp.MustCompile("^\d+$")

// Alphanumeric
regexp.MustCompile("^[a-zA-Z0-9]+$")

Python

import re

# Email
re.compile(r"^[\w\.-]+@[\w\.-]+\.[a-z]{2,}$", re.IGNORECASE)

# Phone (US)
re.compile(r"^\+?1?[-. ]?\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$")

# URL
re.compile(r"https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&\/\/=]*)")

# Digits only
re.compile(r"^\d+$")

# Alphanumeric
re.compile(r"^[a-zA-Z0-9]+$")

Zig

// Use external regex library
// Patterns similar to other languages

C#

using System.Text.RegularExpressions;

// Email
new Regex(@"^[w.-]+@[w.-]+.[a-z]{2,}$", RegexOptions.IgnoreCase);

// Phone (US)
new Regex(@"^+?1?[-. ]?(?([0-9]{3}))?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$");

// URL
new Regex(@"https?://(www.)?[-a-zA-Z0-9@:%._+~#=]{1,256}.[a-zA-Z0-9()]{1,6}([-a-zA-Z0-9()@:%_+.~#?&//=]*)");

// Digits only
new Regex(@"^d+$");

// Alphanumeric
new Regex(@"^[a-zA-Z0-9]+$");

C++

#include <regex>

// Email
std::regex email(R"(^[w.-]+@[w.-]+.[a-z]{2,}$)", std::regex_constants::icase);

// Phone (US)
std::regex phone(R"(^+?1?[-. ]?(?([0-9]{3}))?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$)");

// URL
std::regex url(R"(https?://(www.)?[-a-zA-Z0-9@:%._+~#=]{1,256}.[a-zA-Z0-9()]{1,6}([-a-zA-Z0-9()@:%_+.~#?&//=]*))");

// Digits only
std::regex digits(R"(^d+$)");

// Alphanumeric
std::regex alnum(R"(^[a-zA-Z0-9]+$)");

C

// Use POSIX regex or PCRE
// Patterns similar to other languages
// Escape sequences may differ

Java

import java.util.regex.Pattern;

// Email
Pattern.compile("^[\w\.-]+@[\w\.-]+\.[a-z]{2,}$", Pattern.CASE_INSENSITIVE);

// Phone (US)
Pattern.compile("^\+?1?[-. ]?\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$");

// URL
Pattern.compile("https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&\/\/=]*)");

// Digits only
Pattern.compile("^\d+$");

// Alphanumeric
Pattern.compile("^[a-zA-Z0-9]+$");

Ruby

# Email
/^[w.-]+@[w.-]+.[a-z]{2,}$/i

# Phone (US)
/^+?1?[-. ]?(?([0-9]{3}))?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$/

# URL
/https?://(www.)?[-a-zA-Z0-9@:%._+~#=]{1,256}.[a-zA-Z0-9()]{1,6}([-a-zA-Z0-9()@:%_+.~#?&//=]*)/

# Digits only
/^d+$/

# Alphanumeric
/^[a-zA-Z0-9]+$/

Swift

import Foundation

// Email
try NSRegularExpression(pattern: "^[\w\.-]+@[\w\.-]+\.[a-z]{2,}$", options: .caseInsensitive)

// Phone (US)
try NSRegularExpression(pattern: "^\+?1?[-. ]?\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$")

// URL
try NSRegularExpression(pattern: "https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&\/\/=]*)")

// Digits only
try NSRegularExpression(pattern: "^\d+$")

// Alphanumeric
try NSRegularExpression(pattern: "^[a-zA-Z0-9]+$")

Key Takeaways

• Built-in vs library support — JavaScript (/pattern/ or new RegExp), Python (re module), Ruby (/pattern/), Java (Pattern/Matcher), C# (Regex), C++ (<regex>), and Go (regexp) include regex in the standard library. C requires POSIX regex.h or a library like PCRE; Zig has no built-in regex and typically uses external libraries; Rust needs the regex crate. If you're building a project that heavily uses pattern matching, prefer a language with built-in support; for systems or embedded work, factor in library dependencies.

• Syntax variations — Most languages use PCRE-style patterns. JavaScript and Ruby allow literal syntax: /hello/i for case-insensitive. Go and Rust use string-based patterns: Regex::new(r"(?i)hello") in Rust or regexp.Compile("(?i)hello") in Go. Replace operations also differ: Ruby's "hello hello".gsub(/hello/, "Hi") vs. Python's re.sub(r"hello", "Hi", text). When porting regex between languages, watch for escaping differences—especially in string literals where backslashes may need doubling.

• Capture groups — Named groups (?<first>\w+) (?<last>\w+) are supported in JavaScript, PHP, Rust, Python, C#, Java, and Ruby, giving clean access like match.groups.first or caps["first"]. C's POSIX regex provides regmatch_t for positional groups but not named groups; Swift's NSRegularExpression exposes ranges by index, requiring manual extraction. For parsing structured text (e.g., log lines, config files), prefer a language with named group support to keep code readable.

• API ergonomics — Ruby and JavaScript offer the most concise syntax: "Hello World" =~ /hello/i returns the match index; "hello hello".gsub(/hello/, "Hi") does replace-all in one line. C, C++, and Swift require more boilerplate: opening files, iterating with sregex_iterator, converting NSRange to String indices. For quick text processing and scripting, choose Ruby, Python, or JavaScript; for performance-critical or low-level code, accept the verbosity of C/C++ or the explicit APIs of Rust and Go.