ES2018: s (dotAll) flag for regular expressions

The proposal “s (dotAll) flag for regular expressions” by Mathias Bynens is at stage 4. This blog post explains how it works.

Overview  #

Currently, the dot (.) in regular expressions doesn’t match line terminator characters:

> /^[^]$/.test('\n')
true

The proposal specifies the regular expression flag /s that changes that:

> /^.$/s.test('\n')
true

Limitations of the dot (.) in regular expressions  #

The dot (.) in regular expressions has two limitations.

First, it doesn’t match astral (non-BMP) characters such as emoji:

> /^.$/.test('😀')
false

This can be fixed via the /u (unicode) flag:

> /^.$/u.test('😀')
true

Second, the dot does not match line terminator characters:

> /^.$/.test('\n')
false

That can currently only be fixed by replacing the dot with work-arounds such as [^] (“all characters except no character”) or [\s\S] (“either whitespace nor not whitespace”).

> /^[^]$/.test('\n')
true
> /^[\s\S]$/.test('\n')
true

Line terminators recognized by ECMAScript  #

Line termators in ECMAScript affect:

• The dot, in all regular expressions that don’t have the flag /s.
• The anchors ^ and $ if the flag /m (multiline) is used.

The following for characters are considered line terminators by ECMAScript:

• U+000A LINE FEED (LF) (\n)
• U+000D CARRIAGE RETURN (CR) (\r)
• U+2028 LINE SEPARATOR
• U+2029 PARAGRAPH SEPARATOR

There are additionally some newline-ish characters that are not considered line terminators by ECMAScript:

• U+000B VERTICAL TAB (\v)
• U+000C FORM FEED (\f)
• U+0085 NEXT LINE

Those three characters are matched by the dot without a flag:

> /^...$/.test('\v\f\u{0085}')
true

The proposal  #

The proposal introduces the regular expression flag /s (short for “singleline”), which leads to the dot matching line terminators:

> /^.$/s.test('\n')
true

The long name of /s is dotAll:

> /./s.dotAll
true
> /./s.flags
's'
> new RegExp('.', 's').dotAll
true
> /./.dotAll
false

dotAll vs. multiline  #

• dotAll only affects the dot.
• multiline only affects ^ and $.

FAQ  #

Why is the flag named /s?  #

dotAll is a good description of what the flag does, so, arguably, /a or /d would have been better names. However, /s is already an established name (Perl, Python, Java, C#, ...).