regex alternation — | operator and grouping

Quick Answer

# Match "cat" or "dog"
/cat|dog/

# Anchored alternatives (group with parentheses)
/^(cat|dog)$/   # matches exactly "cat" or "dog"

# Non-capturing group
/(?:cat|dog)s/  # matches "cats" or "dogs", group not captured

Usage

Alternation matches one of several patterns at the same position. Without grouping, | applies to the whole expression on each side.

Other causes & fixes

Alternation has lowest precedence

# "cat or dog" — ^ and $ apply to each alternative
/^cat$|^dog$/   # matches "cat" or "dog" at start/end

# WITHOUT outer group: ^ only applies to "cat"
/^cat|dog$/     # matches "cat" at start OR "dog" at end — NOT what you want

# CORRECT: group the alternatives
/^(cat|dog)$/

Capture vs non-capture groups

// Capturing group — result stored in match[1]
'catfish'.match(/(cat|dog)fish/)
// match[0] = 'catfish', match[1] = 'cat'

// Non-capturing group — no match[1]
'catfish'.match(/(?:cat|dog)fish/)
// match[0] = 'catfish'

Match multiple file extensions

// Match .jpg, .jpeg, .png, .gif
/\.(jpg|jpeg|png|gif)$/i

# In Python
import re
re.search(r'\.(jpg|jpeg|png|gif)$', 'image.PNG', re.IGNORECASE)