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+# **The Sed Mastery Guide: From Basics to Advanced Text Alchemy**
+
+## **Table of Contents**
+1. [Sed Fundamentals](#1-sed-fundamentals)
+2. [Pattern Addressing](#2-pattern-addressing)
+3. [Advanced Substitutions](#3-advanced-substitutions)
+4. [Multi-Command Scripts](#4-multi-command-scripts)
+5. [Branching & Flow Control](#5-branching--flow-control)
+6. [Real-World Recipes](#6-real-world-recipes)
+7. [Performance & Limitations](#7-performance--limitations)
+
+---
+
+## **1. Sed Fundamentals**
+
+### **Core Syntax**
+```bash
+sed [OPTIONS] 'COMMAND' file.txt
+```
+
+### **Essential Operations**
+| Command | Description | Example |
+|---------|-------------|---------|
+| `s/old/new/` | Substitute first occurrence | `sed 's/foo/bar/'` |
+| `s/old/new/g` | Substitute globally | `sed 's/foo/bar/g'` |
+| `p` | Print pattern space | `sed -n '1,5p'` |
+| `d` | Delete lines | `sed '/error/d'` |
+| `i\text` | Insert before line | `sed '3i\New Line'` |
+| `a\text` | Append after line | `sed '$a\END'` |
+
+### **Common Flags**
+```bash
+sed -i '' file.txt       # In-place edit (BSD/macOS)
+sed -i file.txt          # In-place edit (Linux)
+sed -n '1,5p' file.txt   # Print only lines 1-5
+sed -e 'cmd1' -e 'cmd2'  # Multiple commands
+```
+
+---
+
+## **2. Pattern Addressing**
+
+### **Line Number Targeting**
+```bash
+sed '5s/foo/bar/'        # Only line 5
+sed '1,10d'              # Delete lines 1-10
+sed '$d'                 # Delete last line
+```
+
+### **Regex Patterns**
+```bash
+sed '/^#/d'              # Delete comment lines
+sed '/start/,/end/d'     # Delete range between patterns
+sed '/pattern/!d'        # Keep only matching lines
+```
+
+### **Special Addresses**
+```bash
+sed -n '1~2p'            # Every 2nd line starting at 1
+sed -n '2,${p;n}'        # Every other line from line 2
+```
+
+---
+
+## **3. Advanced Substitutions**
+
+### **Capture Groups**
+```bash
+echo "Hello World" | sed 's/\(Hello\) \(World\)/\2 \1/'
+# Output: World Hello
+```
+
+### **Custom Delimiters**
+```bash
+sed 's|/usr/local|/opt|g'  # Use | instead of /
+```
+
+### **Case Conversion**
+```bash
+sed 's/.*/\L&/'           # Lowercase entire line
+sed 's/.*/\U&/'           # Uppercase entire line
+```
+
+### **Conditional Replacement**
+```bash
+sed '/error/s/foo/bar/'   # Only substitute if line contains "error"
+```
+
+---
+
+## **4. Multi-Command Scripts**
+
+### **Command Files**
+```bash
+# script.sed
+/error/ {
+  s/foo/bar/g
+  p
+}
+
+sed -f script.sed file.txt
+```
+
+### **Inline Multi-Commands**
+```bash
+sed -e 's/foo/bar/' -e '/baz/d' file.txt
+```
+
+### **Hold Space Operations**
+```bash
+sed -n '1h; 2,${H;g; s/\n/ /g; p}'  # Combine lines 1-N
+```
+
+---
+
+## **5. Branching & Flow Control**
+
+### **Basic Branching**
+```bash
+sed '/start/{:loop; N; /end/!b loop; s/start.*end/replacement/}'
+```
+
+### **Labels & Loops**
+```bash
+sed ':top; /pattern/{N; b top}'  # Accumulate until pattern fails
+```
+
+### **Conditional Branch**
+```bash
+sed '/error/{s/foo/bar/; t; s/foo/baz/}'  # If first sub worked, skip second
+```
+
+---
+
+## **6. Real-World Recipes**
+
+### **CSV Processing**
+```bash
+sed 's/^\([^,]*\),\([^,]*\)/\2,\1/' data.csv  # Swap first two columns
+```
+
+### **Log File Cleaning**
+```bash
+sed -E '/^(DEBUG|TRACE)/d; s/[0-9]{4}-[0-9]{2}-[0-9]{2}/DATE_REDACTED/g' app.log
+```
+
+### **HTML/XML Processing**
+```bash
+sed '/<script/,/<\/script>/d' page.html  # Remove script tags
+```
+
+### **Code Refactoring**
+```bash
+# Rename function and all calls
+sed -i 's/old_func_name/new_func_name/g' *.py
+```
+
+---
+
+## **7. Performance & Limitations**
+
+### **Large File Handling**
+```bash
+sed -n '1,1000000p' huge.log > sample.log  # Extract first million lines
+```
+
+### **When Not to Use Sed**
+- JSON/XML with nested structures (use `jq/xmllint`)
+- Multi-line records spanning variable lines (use `awk`)
+- Binary file editing (use `xxd + sed + xxd -r`)
+
+### **Performance Tips**
+```bash
+LC_ALL=C sed ...        # 2-3x speedup for ASCII
+sed ':a;N;$!ba;s/\n//g' # Slurp entire file (careful with memory)
+```
+
+---
+
+## **Pro-Level Techniques**
+
+### **In-Place Editing Backup**
+```bash
+sed -i.bak 's/foo/bar/' file.txt  # Creates file.txt.bak
+```
+
+### **Non-Greedy Matching**
+```bash
+sed 's/foo\(.*\?\)bar/\1/'  # Perl-style regex (requires -r/-E)
+```
+
+### **Random Line Selection**
+```bash
+sed -n "$RANDOM"' {p;q}'    # Get one random line
+```
+
+---
+
+## **Sed + Awk Power Combo**
+```bash
+# Extract columns from lines matching pattern
+sed -n '/pattern/p' file | awk '{print $2,$4}'
+```
+
+For complex text transformations, consider combining sed with:
+- `awk` for columnar data
+- `perl` for advanced regex
+- `grep` for filtering
+
+**Need a custom sed solution?** Provide your text transformation challenge!