Regex Anchors & Boundaries Guía rápida

Match positions in text with anchors and word boundaries.

What this cheat sheet is for

This regex anchors & boundaries reference helps you complete common developer tasks quickly and avoid repeated trial-and-error when working under deadlines.

When you need a reliable command or pattern during debugging and incident response.
When you are implementing a feature and want a known-good syntax example immediately.
When onboarding teammates and documenting repeatable workflows for daily use.
When reviewing pull requests and verifying that command usage follows best practices.

Line Anchors

Match at line start or end.

^           # Start of line
$           # End of line
^abc        # Line starts with "abc"
xyz$        # Line ends with "xyz"
^abc$       # Entire line is "abc"

Word Boundaries

Match at word boundaries.

\b          # Word boundary
\B          # Non-word boundary
\bcat\b     # Match "cat" as whole word
\Bcat\B     # Match "cat" not at boundaries

Start of String

Match beginning of string.

^           # Start of string (in multiline mode: line)
\A          # Start of string (always)

End of String

Match end of string.

$           # End of string (in multiline mode: line)
\Z          # End of string (always)
\z          # Absolute end of string

Word Boundary Ejemplos

Matching whole words.

\bword\b     # Match "word" as whole word
\btest       # Match "test" at start of word
test\b       # Match "test" at end of word
\b\w{4}\b    # Match any 4-letter word

Line Anchor Ejemplos

Match at specific line positions.

^\d          # Line starts with digit
\.$          # Line ends with period
^$           # Empty line
^\s*$        # Blank line (only whitespace)

Combining Anchors

Use multiple anchors together.

^\w+$        # Entire line is word characters
^https?://   # Line starts with http:// or https://
\.com$       # Line ends with .com
^\d{3}-\d{4}$ # Entire line is phone format

Practical Ejemplos

Real-world anchor usage.

^\#          # Line starts with # (markdown heading)
^import      # Python import at line start
;\s*$        # Line ends with semicolon
^\s+         # Leading whitespace
\s+$         # Trailing whitespace

Common mistakes / Errores comunes

People often copy a command or pattern without adapting placeholders, which can break production workflows unexpectedly.
It is easy to forget environment-specific differences, so always verify behavior in your shell, runtime, or API gateway before shipping.
Many errors come from skipping small validation steps, so test with realistic sample input before relying on the result.

Last updated: February 2026