How to Use Regex: Practical Guide to Regular Expressions

Regular expressions are used in two main contexts: text editors/command-line tools (for searching and replacing) and programming languages (for validation and extraction). This guide covers both.

In Python: the re module

import re

text = "The price is $42.99. Sale ends 2026-04-30."

Test if pattern exists:

if re.search(r'\$[\d.]+', text):
    print("Price found")

Find first match:

match = re.search(r'\$[\d.]+', text)
if match:
    print(match.group())   # "$42.99"
    print(match.start())   # 13 (start position)
    print(match.end())     # 19
    print(match.span())    # (13, 19)

Find all matches:

prices = re.findall(r'\$[\d.]+', text)
print(prices)  # ['$42.99']

# With groups, returns list of tuples
dates = re.findall(r'(\d{4})-(\d{2})-(\d{2})', text)
print(dates)  # [('2026', '04', '30')]

Replace:

result = re.sub(r'\$[\d.]+', 'PRICE', text)
# "The price is PRICE. Sale ends 2026-04-30."

# Replace with function
result = re.sub(r'\$[\d.]+', lambda m: m.group().upper(), text)

# Limit replacements
result = re.sub(r'\d', 'X', text, count=3)

Split:

words = re.split(r'\W+', "Hello, World! How are you?")
print(words)  # ['Hello', 'World', 'How', 'are', 'you', '']

Compile for reuse (performance):

pattern = re.compile(r'\$[\d.]+', re.IGNORECASE)
match = pattern.search(text)
all_matches = pattern.findall(text)

Common flags:

re.IGNORECASE   # or re.I: case-insensitive
re.MULTILINE    # or re.M: ^ and $ match line boundaries
re.DOTALL       # or re.S: . matches newlines
re.VERBOSE      # or re.X: allow comments and whitespace in pattern

In JavaScript

const text = "The price is $42.99. Sale ends 2026-04-30.";

Test:

/\$[\d.]+/.test(text);  // true

Match (first):

const match = text.match(/\$[\d.]+/);
// ["$42.99", index: 13, input: ..., groups: undefined]

Match all:

const allPrices = text.match(/\$[\d.]+/g);
// ["$42.99"]

// matchAll returns iterator with full match objects
for (const match of text.matchAll(/(\d{4})-(\d{2})-(\d{2})/g)) {
  console.log(match[1], match[2], match[3]);  // "2026" "04" "30"
}

Replace:

text.replace(/\$[\d.]+/, 'PRICE');
// Replaces first only

text.replace(/\$[\d.]+/g, 'PRICE');
// Replaces all (with g flag)

text.replace(/(\d{4})-(\d{2})-(\d{2})/g, '$3/$2/$1');
// Swap date format: "2026-04-30" → "30/04/2026"

Split:

"Hello, World! How are you?".split(/\W+/);
// ["Hello", "World", "How", "are", "you", ""]

In the command line

grep:

# Basic search
grep 'error' logfile.log

# Case-insensitive
grep -i 'error' logfile.log

# Extended regex (allows + ? | etc. without escaping)
grep -E '\d{4}-\d{2}-\d{2}' logfile.log

# Show only matching part (not full line)
grep -oE '\$[\d.]+' prices.txt

# Recursive search
grep -r 'TODO' ./src/

# Invert (show non-matching lines)
grep -v 'DEBUG' logfile.log

sed (stream editor):

# Replace first occurrence per line
sed 's/old/new/' file.txt

# Replace all occurrences per line
sed 's/old/new/g' file.txt

# Case-insensitive replace
sed 's/old/new/gi' file.txt

# Delete lines matching pattern
sed '/pattern/d' file.txt

# Print only matching lines
sed -n '/pattern/p' file.txt

# In-place edit (modify file)
sed -i 's/old/new/g' file.txt

In VS Code

Search with regex in VS Code:

1. Open Find: Ctrl+F (or Cmd+F on Mac)
2. Click the .* button to enable regex mode
3. Type your regex pattern

For Find and Replace:

1. Open Replace: Ctrl+H (or Cmd+H)
2. Enable regex with the .* button
3. Use $1, $2 for group references in the replace field

Example: convert snake_case to camelCase:

• Find: _(\w)
• Replace: \U$1 (in some editors) or handle with a script

In grep/VS Code: useful patterns for files

# Find all TODO comments
grep -rn 'TODO\|FIXME\|HACK' ./src/

# Find files with debug prints
grep -rn 'console\.log\|print(' ./src/ --include="*.js" --include="*.py"

# Find lines with long string literals
grep -En '"[^"]{100,}"' ./src/*.py

# Find all email addresses in a file
grep -Eo '[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}' contacts.txt

Build and test regex patterns at regexbuilder.io.