In JavaScript, does an empty regex pattern have defined behavior?

Question

var pattern = /(?:)/

From my testing, it seems to match everything. Is this the defined behavior?

@mini: That isn't true. The reverse question has come up and gotten lots of attention. — SLaks
– SLaks, Commented May 23, 2011 at 1:29
I could imagine a system where the user is expected to supply a regex string for matching "something" and his code encapsulates that regex into a (?:...), and then evals it. In fact somebody asked about this case not long ago; they were letting people supply regexes at runtime. — Cheeso
– Cheeso, Commented May 23, 2011 at 1:29
Both /.^/ and /$./ mismatch any string. Note that /$^/ still matches the empty string because the beginning is also the ending. — jdh8
– jdh8, Commented Aug 2, 2017 at 20:54

user113716 · Accepted Answer · 2011-05-23 01:51:03Z

18

This doesn't directly answer the question, but here's what the spec has to say about the empty regular expression:

From 15.5.4.14 String.prototype.split (separator, limit)

The value of separator may be an empty String, an empty regular expression, or a regular expression that can match an empty String.

And from 7.8.5 Regular Expression Literals

NOTE Regular expression literals may not be empty; instead of representing an empty regular expression literal, the characters // start a single-line comment. To specify an empty regular expression, use: /(?:)/ .

So given that it is an accepted value for the separator in .split(), I would guess that it is the defined behavior as a way to split on every character.

"fjeij;als#%^&é.\n isoij\t;oi`1=+-]\r".split(/(?:)/);

["f", "j", "e", "i", "j", ";", "a", "l", "s", "#", "%", "^", "&", "é", ".", "
", " ", "i", "s", "o", "i", "j", "  ", ";", "o", "i", "`", "1", "=", "+", "-", "]", "
"]

edited May 23, 2011 at 1:51

answered May 23, 2011 at 1:37

user113716

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2 Comments

ClosureCowboy Over a year ago

Thank you! This seems reasonably defined, because it would be odd for patterns to match different strings in different contexts.

gaRex Over a year ago

Try in console new RegExp() :))

Cheeso · Accepted Answer · 2011-05-23 01:28:35Z

9

/(?:)/ matches "nothing", which matches everything. There is nothing in everything. Heh heh.
Yes, I would expect this.

answered May 23, 2011 at 1:28

Cheeso

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