Can't find anything about spaces in the official Emacs docs [duplicate]

Question

I want to match every string that cointains a year and a month name e.g. like that:

2022 Jul or 2022 Jul (and more spaces than 2) or even 2022Jul.

The regex I have so far is:

"\\([0-9]\\{4\\}\\)"

And this will grab the year.

But now I don't know how to target the existence of spaces.

In JavaScript it would be something like:

[0-9]{4}\s*[A-Za-z]{3}

But I can't make it work in Emacs for anything ;(.

What strikes me the most is that there is ZERO!!! mention about spaces or whitespaces or breaks in the offical Emacs docs. Check for yourself: https://www.gnu.org/software/emacs/manual/html_node/elisp/Regexp-Special.html

And searching on Google doesn't bring anything useful.

Can you guys help how to target spaces in my regex example?

Can anybody help? I don't get the Regex in Emacs at all.

Answer 1

Emacs uses \scode to target backslash constructs. See more in the documentation here: https://www.gnu.org/software/emacs/manual/html_node/elisp/Regexp-Backslash.html

In particular, the part in the description about whitespace may help you get the selection you are after:

‘\scode’ ¶

    matches any character whose syntax is code. Here code is a character that represents a syntax code: thus, ‘w’ for word constituent, ‘-’ for whitespace, ‘(’ for open parenthesis, etc. To represent whitespace syntax, use either ‘-’ or a space character. See Table of Syntax Classes, for a list of syntax codes and the characters that stand for them.

Try using \s- to select the whitespace here.

More information on Syntax Classes (as referenced in docstring above) available here: https://www.gnu.org/software/emacs/manual/html_node/elisp/Syntax-Class-Table.html

In particular, this could be useful for understanding whitespace selection:

36.2.1 Table of Syntax Classes

Here is a table of syntax classes, the characters that designate them, their meanings, and examples of their use.

Whitespace characters: ‘ ’ or ‘-’

    Characters that separate symbols and words from each other. Typically, whitespace characters have no other syntactic significance, and multiple whitespace characters are syntactically equivalent to a single one. Space, tab, and formfeed are classified as whitespace in almost all major modes.

    This syntax class can be designated by either ‘ ’ or ‘-’. Both designators are equivalent.

Answer 2

In addition to \scode, there are also the character classes [:space:] and [:blank:]. See chapter 35.3.1.2 Character Classes for the details. Note especially the first paragraph, where it mentions that character classes go inside square brackets, so you end up with double square brackets when using these.

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You can also just match the space character directly; * will match zero or more spaces (but not other types of whitespace, which you might want to allow)

You may also find M-x re-builder useful. Put some sample text you want to match (and some that you don’t want your regex to match!) into a buffer, then call re-builder. Type in your regex and it will show you what parts of the text in the buffer match. Very handy!