Anchors & Boundaries

^pattern

JS:  /^#!/.test('#!/bin/bash')    // true
JS:  /^#!/.test('echo #!/bin')   // false
Py:  bool(re.match(r'^#!', '#!/usr/bin/env python'))  # True

true, false, True

Note Without the m flag, ^ matches only the very beginning of the entire string. With the m flag, it matches the start of each line (after every newline character).

pattern$

JS:  /\.py$/.test('script.py')       // true
JS:  /\.py$/.test('script.py.bak')   // false
Py:  bool(re.search(r'\.py$', 'main.py'))  # True

true, false, True

Note Without the m flag, $ matches the very end of the string (or before a trailing newline in Python). With m, it matches the end of each line.

\b  matches the boundary between \w and \W (or string start/end)

JS:  'reinvent invent invention'.match(/\binvent\b/g)
Py:  re.findall(r'\binvent\b', 'reinvent invent invention')

["invent"]  (only the standalone word)

Note A word boundary exists at three positions: before the first \w character, after the last \w character, and between a \w and \W character. It does NOT match any characters itself -- it matches a position.

\A  matches the very start of the string (ignores multiline)

Py:  re.search(r'\AFirst', 'First line\nSecond line', re.MULTILINE)
     # Matches 'First'
Py:  re.search(r'^Second', 'First line\nSecond line', re.MULTILINE)
     # Also matches (^ respects multiline)

\A always means absolute start, even with re.MULTILINE

Note \A is not available in JavaScript. It is equivalent to ^ when multiline mode is off, but unlike ^, it never changes behavior with the m flag. Use \A in Python when you always mean the beginning of the entire string.

\Z  matches the very end of the string (ignores multiline)

Py:  re.search(r'end\Z', 'start\nmiddle\nend')
     # Matches 'end'
Py:  re.search(r'middle$', 'start\nmiddle\nend', re.MULTILINE)
     # Also matches ($ respects multiline)

\Z matches only the absolute end, regardless of flags

Note In Python, \Z matches the absolute end (after the last character). Note: \z (lowercase) is used in some other flavors (Ruby, PCRE) for the same purpose, while Python's \Z (uppercase) does not match before a trailing newline. Not available in JavaScript.

Use m flag to make ^ and $ match line boundaries

JS:  'line1\nline2\nline3'.match(/^\w+/gm)
Py:  re.findall(r'^\w+', 'line1\nline2\nline3', re.MULTILINE)

["line1", "line2", "line3"]  (each line start matched)

Note Without the m flag, only 'line1' would match ^\w+. A common mistake is forgetting the m flag when processing multi-line text like log files or configuration files. In Python, re.MULTILINE or re.M is the flag; in JS, use /m.