Webflags: g

HTML Tag Matcher

Match paired HTML tags and capture the tag name and inner content using a back-reference.

Available in

JavaScript / ECMAScript code

jsJavaScript

const re = new RegExp("<([a-zA-Z][a-zA-Z0-9]*)\\b[^>]*>([\\s\\S]*?)<\\/\\1>", "g");
const input = "<p>Hello world</p>";
const matches = [...input.matchAll(re)];
console.log(matches.map(m => m[0]));

Uses `String.prototype.matchAll` for global iteration (Node 12+ / all modern browsers).

Python (re) code

pyPython

import re

pattern = re.compile(r"<([a-zA-Z][a-zA-Z0-9]*)\b[^>]*>([\s\S]*?)<\/\1>")
input_text = "<p>Hello world</p>"
for m in pattern.finditer(input_text):
    print(m.group(0))

Stdlib `re` module — no third-party dependency. Works on Python 3.6+.

Why it doesn't work in GO

Go's RE2 engine doesn't support backreferences (`\1`, `\2`, …) for the same linear-time reason.

Approach

Match the candidate substring with a single capture, then verify the duplication in code; or use JS / Python which both support backreferences.

Read the full guide →

Workaround code in Go (RE2)

goGo (workaround)

package main

import (
	"fmt"
	"regexp"
)

// RE2 doesn't support backreferences. The fix: capture the candidate
// substring once, then verify the duplication in Go code.
//
// Example: instead of `(\w+)\s+\1` (duplicate words), capture two
// adjacent words and compare them.

func main() {
	re := regexp.MustCompile(`\b(\w+)\s+(\w+)\b`)
	input := "the cat cat ran ran fast"
	for _, m := range re.FindAllStringSubmatch(input, -1) {
		if m[1] == m[2] { // the backreference equality, in code
			fmt.Println(m[0])
		}
	}
}

Capture the candidate substrings as separate groups, then compare them in Go code instead of via a backreference.

Pattern

regexengine-agnostic

<([a-zA-Z][a-zA-Z0-9]*)\b[^>]*>([\s\S]*?)<\/\1>   (flags: g)

Raw source: <([a-zA-Z][a-zA-Z0-9]*)\b[^>]*>([\s\S]*?)<\/\1>

How it works

Group 1 captures the tag name. [^>]* matches attributes. [\s\S]*? lazily captures inner content. \1 back-references the opening tag name to ensure the closing tag matches.

Examples

Input

<p>Hello world</p>

Matches

<p>Hello world</p>

Input

<div class="box">content</div>

Matches

<div class="box">content</div>

Common use cases

•Basic HTML parsing and extraction
•Template content replacement
•Static site content scraping
•Email template processing

Related patterns

HTML Attribute

Web

Match HTML attributes of the form name="value" or name='value' and capture both parts.

HTML Comment

Web

Match HTML comments, including multi-line comments and empty ones.

HTML Entity

Web

Match HTML entities in named (`&`), numeric (`{`), or hex (`💩`) form.

Cookie Header Value

Web

Parse name=value pairs from an HTTP `Cookie:` header value.

Related concepts

Custom Character Sets: [abc], [a-z], [^abc]

Concept

Square brackets build a custom character class. [abc] matches any of a, b, or c. [a-z] is a range. A leading ^ negates the set.

Lazy vs. Greedy Quantifiers

Concept

Greedy quantifiers (*, +) consume as much as possible before backtracking. Lazy quantifiers (*?, +?) consume as little as possible.

Capturing Groups and Non-Capturing Groups

Concept

Parentheses group tokens and capture the matched substring. (?:...) groups without capturing — use it when you want grouping for quantifiers or alternation but don't need the submatch.

Regex Flags in JavaScript: g, i, m, s, u, y

Concept

Flags modify regex behavior globally. g enables global matching, i makes it case-insensitive, m changes anchor behavior, s dots match newlines, u enables Unicode, y is sticky.

How to Match Across Newlines in JavaScript

How-to

The dot doesn't match newlines by default. Use the s (dotall) flag, or build an explicit [\s\S] alternative for engines that predate s.

How to Escape Regex Special Characters

How-to

Backslash-escape any of .^$*+?()[]{}|\ to match them literally. When building a regex from user input, use a full-escape helper function.

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