regex anchors — ^ and $ match start and end

Q: \A and \Z — always match string boundaries (Python)

\A and \Z always match the start and end of the whole string, even with re.MULTILINE. ``` import re text = "line1\nline2" re.findall(r'^\w+', text, re.MULTILINE) # ['line1', 'line2'] — ^ per line re.findall(r'\A\w+', text, re.MULTILINE) # ['line1'] — always string start ```

Quick Answer

// Match entire string
/^hello$/.test('hello')        // true
/^hello$/.test('say hello')    // false
/^hello$/.test('hello world')  // false

# Python
import re
re.fullmatch(r'hello', 'hello')          # matches
re.match(r'^hello$', 'hello')            # matches
re.match(r'^hello$', 'hello world')      # None

Usage

Without anchors, a pattern can match anywhere in the string. Use ^ and $ to constrain where the match occurs.

Other causes & fixes

Validate entire input

// JavaScript — validate hex color
/^#[0-9a-fA-F]{6}$/.test('#ff5733')   // true
/^#[0-9a-fA-F]{6}$/.test('#ff5733xx') // false

# Python — validate US zip code
bool(re.fullmatch(r'\d{5}(-\d{4})?', '12345'))       # True
bool(re.fullmatch(r'\d{5}(-\d{4})?', '12345-6789'))   # True

\A and \Z — always match string boundaries (Python)

\A and \Z always match the start and end of the whole string, even with re.MULTILINE.

import re
text = "line1\nline2"
re.findall(r'^\w+', text, re.MULTILINE)   # ['line1', 'line2'] — ^ per line
re.findall(r'\A\w+', text, re.MULTILINE) # ['line1'] — always string start

Multiline mode changes ^ and $ behavior

// In multiline mode, ^ and $ match per line
const lines = "first\nsecond\nthird";
lines.match(/^\w+$/gm)   // ['first', 'second', 'third']