Anchors — ^ and $ · RegEx Quest

Pin the match to a position

Anchors are zero-width — they don't consume characters, they just assert that you're at a specific position.

^ — start of string (or start of line, with the MULTILINE flag)
$ — end of string (or end of line, with MULTILINE)

Without anchors, your pattern can match anywhere. With ^pattern$, you're saying "the entire input is exactly this pattern."

The MULTILINE flag

By default, ^ and $ match the start and end of the whole string. With re.MULTILINE (Python) or the m flag (PCRE/JS), they additionally match at the start and end of every line. This matters for processing multi-line input — log files, multi-line user input, etc.

The fullmatch alternative

In Python, re.fullmatch(pat, text) is shorthand for re.match(r"^" + pat + r"$", text). Use it when you want to validate that the entire string matches a pattern. It's clearer than wrapping anchors and harder to mess up.

The \A and \Z anchors

Some flavors (Python, PCRE, .NET) provide \A and \Z as absolute string-start and string-end. They ignore MULTILINE. So \A always means "start of input," never "start of line." Useful when you want anchoring that doesn't change with flags.

Code

Anchors in practice·python

import re

# 'foo' anywhere
re.search(r'foo', 'foobar foo').group()  # 'foo'

# 'foo' as the entire string?
bool(re.fullmatch(r'foo', 'foo'))     # True
bool(re.fullmatch(r'foo', 'foobar'))  # False

# Per-line with MULTILINE
log = 'line one\nERROR: oops\nline three'
re.findall(r'^ERROR.*$', log, re.MULTILINE)
# ['ERROR: oops']

# Without MULTILINE — only matches if ERROR is at very start
re.findall(r'^ERROR.*$', log)
# []

Anchors — ^ and $

Pin the match to a position

The MULTILINE flag

The fullmatch alternative

The \A and \Z anchors

Code

External links

Exercise

Progress

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