Capture Groups, Backreferences & Substitution
Use parentheses to capture parts of a match and reuse them with backreferences for powerful matching and text substitution.
Capture Groups, Backreferences & Substitution is a free DevOps Bootcamp lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the DevOps Bootcamp learning path, and your progress syncs across the web and the CoddyKit app. The DevOps Bootcamp course includes 4 lessons in total.
What Are Capture Groups?
Parentheses ( ) create a capture group that remembers the text it matched so you can reference it later.
echo "2026-05-29" | grep -E "([0-9]{4})-([0-9]{2})-([0-9]{2})"Numbering of Groups
Groups are numbered left to right by their opening parenthesis. Group 1 is the first (, group 2 the second, and so on.
# (year)(month)(day)
# group 1 = year, 2 = month, 3 = dayBackreferences in Matching
A backreference like \1 matches the SAME text a previous group captured. Great for finding duplicates.
echo "hello world world" | grep -E "(\w+) \1"Detecting Doubled Words
The pattern (\w+) \1 finds a word followed by itself, a common typo.
echo "the the cat sat" | grep -oE "(\w+) \1"Capture Groups with sed
In sed substitutions, captured groups are reused in the replacement as \1, \2. Use -E for extended regex.
echo "2026-05-29" | sed -E "s|([0-9]{4})-([0-9]{2})-([0-9]{2})|\3/\2/\1|"Swapping Fields
Capture two fields and reorder them in the replacement to transform data formats.
echo "Doe, John" | sed -E "s/(\w+), (\w+)/\2 \1/"Non-Capturing Concept
Sometimes you group only to apply a quantifier, not to capture. In PCRE you can write (?:...) to avoid numbering.
# PCRE only (grep -P):
echo "abcabc" | grep -P "(?:abc){2}"Alternation Inside Groups
Combine groups with alternation | to match one of several options while still capturing the result.
echo "cat dog fish" | grep -oE "(cat|dog)"Quantifying a Group
Apply quantifiers to a whole group to repeat a pattern. (ab)+ matches one or more ab sequences.
echo "ababab" | grep -oE "(ab)+"Capturing in awk
awk uses match() with RSTART and RLENGTH to extract the matched portion.
echo "id=42" | awk "{ if (match(\$0, /id=[0-9]+/)) print substr(\$0, RSTART, RLENGTH) }"Practical Reformatting
Capture groups shine when reformatting logs or CSV columns into a new layout in a single pass.
echo "John,30,NYC" | sed -E "s/([^,]+),([^,]+),([^,]+)/Name: \1 Age: \2 City: \3/"Quick Check
Test your capture group knowledge.
Recap
You mastered capture groups:
- Parentheses capture and number matched text
- Backreferences
\1match the same text again - In
sed, reuse groups in replacements to reorder fields - Quantify and alternate inside groups for flexible patterns
Learn DevOps Bootcamp with an AI tutor — free
Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.
- Courses
- 142
- Lessons
- 568
Frequently Asked Questions
Is the “Capture Groups, Backreferences & Substitution” lesson free?
Yes — any 3 lessons of the DevOps Bootcamp learning path, including “Capture Groups, Backreferences & Substitution”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The DevOps Bootcamp course includes 4 lessons in total.
What will I learn in “Capture Groups, Backreferences & Substitution”?
Use parentheses to capture parts of a match and reuse them with backreferences for powerful matching and text substitution. You practise DevOps Bootcamp with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start DevOps Bootcamp?
No prior experience is required. DevOps Bootcamp on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.
How long does the “Capture Groups, Backreferences & Substitution” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this DevOps Bootcamp lesson?
Yes. Every DevOps Bootcamp lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Introduction to Regular Expressions (Regex)
- Advanced Grep with Regex
- Combining Regex with sed/awk
- Capture Groups, Backreferences & Substitution