Javascript regex to match a regex

Question

I'm working on a special regex to match a javascript regex.

For now I have this regex working:

/\/(.*)?\/([i|g|m]+)?/

For example:

'/^foo/'.match(/\/(.*)?\/([i|g|m]+)?/) => ["/^foo/", "^foo", undefined]
'/^foo/i'.match(/\/(.*)?\/([i|g|m]+)?/) => ["/^foo/i", "^foo", "i"]

Now I need to get this regex working with:

'^foo'.match(/\/(.*)?\/([i|g|m]+)?/) => ["^foo", "^foo", undefined]

Unfortunately my previous regex doesn't work for that one.

Can someone help me to find a regex matching this example (and others too):

'^foo'.match([a regex]) => ["^foo", "^foo", undefined]

I literally just asked the question with a new twist. Take a look at slak's comment... — user3186555
– user3186555, Commented May 31, 2016 at 2:28
see also: Is there a regular expression to detect a valid regular expression? — starball
– starball ♦, Commented Sep 20, 2023 at 22:49

Niklas · Accepted Answer · 2015-11-24 11:07:29Z

27

A regular expression to match a regular expression is

/\/((?![*+?])(?:[^\r\n\[/\\]|\\.|\[(?:[^\r\n\]\\]|\\.)*\])+)\/((?:g(?:im?|mi?)?|i(?:gm?|mg?)?|m(?:gi?|ig?)?)?)/

To break it down,

\/ matches a literal /
(?![*+?]) is necessary because /* starts a comment, not a regular expression.
[^\r\n\[/\\] matches any non-escape sequence character and non-start of character group
\[...\] matches a character group which can contain an un-escaped /.
\\. matches a prefix of an escape sequence
+ is necessary because // is a line comment, not a regular expression.
(?:g...)? matches any combination of non-repeating regular expression flags. So ugly.

This doesn't attempt to pair parentheses, or check that repetition modifiers are not applied to themselves, but filters out most of the other ways that regular expressions fail to syntax check.

If you need one that matches just the body, just strip off everything else:

/(?![*+?])(?:[^\r\n\[/\\]|\\.|\[(?:[^\r\n\]\\]|\\.)*\])+/

or alternatively, add "/" at the beginning and end of your input.

edited Nov 24, 2015 at 11:07

Niklas

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answered Jul 24, 2013 at 20:01

Mike Samuel

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9 Comments

Mark Amery Over a year ago

I don't know by looking whether this works, but I do know that it frightens me.

julesbou Over a year ago

'/foo/i'.match(/\/(?![*+?])(?:[^\r\n\[/\\]|\\.|\[(?:[^\r\n\]\\]|\\.)*\])+\/(?:g(?:im?|m)?|i(?:gm?|m)?|m(?:gi?|i)?)?/) result is ['/foo/i']. That's not expected result.

Mike Samuel Over a year ago

@jules, I added parentheses in the appropriate places so that the match has the bits you expect.

Gor Over a year ago

"x = 2; y = x / 3; z = x/4". In this case your regex would not work

scagood Over a year ago

Oh, on second thoughts, if people want regex that matches with a string (or something) after it this: (?:i(?!\w*i)|g(?!\w*g)|m(?!\w*m))*(?!\w) may be more appropriate. (or ((?:([igm])(?!\w*\3))*)(?!\w))

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