Understanding Vim Regular Expressions: A Comparison with PCRE

Zettel#92

zettelkasten
Published

2023-05-31

If you’re reading this in raw Markdown, be careful with the escaped | characters. I had to escape them in the raw text so that they would render in HTML correctly.

Differences Between Vim and PCRE Syntax

Description PCRE Syntax Vim Syntax
Zero or more of previous item, as few as possible (non-greedy) *? \{-}
One or more of previous item, as many as possible (greedy) + \+
One or more of previous item, as few as possible (non-greedy) +? \{-1,}
Zero or one of previous item ? \?
Word boundary \b \< or \>
Group start ( \(
Group end ) \)
Or \| \\|
Escaping special characters \ \\

Examples

The following examples illustrate the differences between PCRE and Vim regex syntax through practical use cases:

Task PCRE Example Vim Example
Match “test” exactly test test
Match “test” at the start of line ^test ^test
Match “test” at the end of line test$ test$
Match “test” as a whole word \btest\b \<test\>
Match one or more “a”s a+ a\+
Match zero or more “a”s a* a*
Match “a” zero or one times a? a\?
Non-greedy match for one or more “a”s a+? a\{-1,}
Non-greedy match for zero or more “a”s a*? a\{-}
Match “a” followed by “b” or “c” a(b\|c) a\(b\\|c\)

These examples should give you an idea of how to convert PCRE regular expressions to Vim’s syntax. Remember that the behavior of some regex patterns can change based on Vim’s magic mode setting, so be sure to specify the mode (\v for very magic and \V for very nomagic) to avoid confusion.

Vim’s Built-in Documentation

:help perl-pattern

This command navigates to a section in Vim’s help documentation that explicitly describes the differences between Vim’s regex and Perl’s regex (PCRE).

Vim Magic Regex Mode

Vim’s regular expressions offer two primary modes: magic and nomagic. These modes define how special characters, those with meanings beyond their literal representation, are treated.

The magic mode aligns Vim’s regular expressions more closely with the behavior of regular expressions in other languages and tools, considering characters such as . , * , ^ , and $ as special. This mode is activated by initiating the regular expression with \v.

In contrast, nomagic mode necessitates escaping more characters with a backslash, as it considers fewer characters special. This mode can be activated by initiating the regular expression with \V.

Importantly, Vim operates in a somewhat magic mode by default, a middle ground between the two extremes. To avoid confusion, it is recommended to explicitly specify the mode (\v for very magic and \V for very nomagic).

Examples of Magic, Somewhat Magic, and Nomagic Modes

The following examples demonstrate the impact of magic, somewhat magic (default), and nomagic modes on Vim regular expressions:

Task Magic Mode Example (\v) Default (Somewhat Magic) Mode Example Nomagic Mode Example (\V)
Match “test” exactly \vtest test \Vtest
Match “test” at the start of line \v^test ^test \V\^test
Match “test” at the end of line \vtest$ test$ \Vtest\$
Match “test” as a whole word \v<test> \<test\> \V\<test\>
Match one or more “a”s \va+ a\+ \Va\+
Match zero or more “a”s \va* a* \Va\*
Match “a” zero or one times \va? a\? \Va\?
Non-greedy match for one or more “a”s \va{-1,} a\{-1,\} \Va\{-1,\}
Non-greedy match for zero or more “a”s \va{-} a\{-\} \Va\{-\}
Match “a” followed by “b” or “c” \v(a\|b) \(\a\|b\) \V\(a\\|b\)

In magic mode, Vim’s regex behaves more like other languages’ regular expressions, so you often need fewer backslashes. The default somewhat magic mode treats fewer characters as special, so more backslashes are required compared to magic mode. nomagic mode treats even fewer characters as special, so even more backslashes are typically required.

Vim’s Built-in Documentation

For more information about the magic mode in Vim’s regular expressions, refer to the built-in documentation by typing:

:help /\v

References