CODEWITHSUNDEEP
javascriptregexwhitespace
Flmhdfj
Flmhdfj

Reputation: 918

weird white spaces in javascript regex

I saw this weird regex in javascript.

string = "abababababababa";
string=string.replace(/ ?a ?/g,"X");
alert(string);

I ran it , and the output I got was all a replaced by X What is puzzling is the white spaces in the regex. I delete the first white space, the script won't run. I delete the 2nd white space, I get one "a" replaced by two "X"s. I wonder how it works.

Upvotes: 4

Views: 612

Answers (3)

hwnd
hwnd

Reputation: 70742

' ' is matching an actual space character. The ? quantifier means Match 1 or 0 times.

Your regular expression does the following:

 ?          ' ' optional space character
a           match 'a'
 ?          ' ' optional space character

It will match the following cases:

  • 'a'
  • 'a '
  • ' a'
  • ' a '

You regular expression will match abababababababa in your input string using the g (global modifier). Meaning all matches (don't return on first match)

  • If you remove the space character at the beginning the preceding token is not quantifiable so it will fail.

  • If you leave the space character but remove the first ? it will fail because it looks to match a space character, however a space character does not exist in the input.

  • If you remove the second space character, it still succeeds the match making a optional here.

Upvotes: 4

Jonathan Lonowski
Jonathan Lonowski

Reputation: 123563

The space actually means to match a space character (U+0020).

The key is the ? quantifier that follows each of them, allowing the pattern to match "0 or 1 spaces" for each, essentially making them optional.

So, the / ?a ?/ pattern is capable of matching:

  • "a"
  • "a "
  • " a"
  • " a "

And, attempting to remove either space will change the meaning of the pattern:

  • Removing the leading space (/?a ?/g) actually results in a SyntaxError as quantifiers require something before them to quantify.

  • Removing the trailing space (/ ?a?/g) is syntactically valid, but the ? quantifier will then apply to the a, changing the possible matches to:

    • ""
    • "a"
    • " "
    • " a"

Upvotes: 9

Austin Brunkhorst
Austin Brunkhorst

Reputation: 21130

the ? character makes the spaces optional in matching.

For example, these strings will be replaced with an X with that regular expression.

> "a"
> " a"
> " a "
> "a "

You can find more information about this quantifier here.

Upvotes: 4

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