What does the regex "/\\*{2,}/" mean? [duplicate] - regex

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Reference - What does this regex mean?
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Closed 7 years ago.
I'm kinda new to regex, and specifically, I don't understand there are 2 backslashes? I mean, I know the second one is to escape the character "*", but what does the first backslash do?
Well I'm passing this regex expression to the php function preg_match(), and I'm trying to find strings that include 2 or more consecutive "*".

Answer 1

That regex is invalid syntax.
You have this piece:
*{2,}
Which basically would read: match n-times, 2 or more times.
The following regex:
/\\*.{2,}/
Is the simplest and closest regex to the one you have, which would read as:
match 0 or more '\' and 2 or more characters that aren't newlines
If you are talking about the string itself, is may be interpreted as 2 things:
/\\*{2,}/
Read as: match a single \ and another \ n-times 2 times or more
This is invalid syntax
/\*{2,}\
Read as match 2 or more *
This is valid syntax
It all varies, depending on the escape character.
Edit:
Since the question was updated to show which language and engine it is being used, I've updated to add the following information:
You have to pass the regex as '/\*{2,}/' OR as "/\\*{2,}/" (watch the quotes).
Both are very similar, except that single quotes ('') only support the following escape sequences:
\' - Produces '
\\- Produces \
Double-quoted strings are treated differently in PHP. And they support almost any escape sequence, like:
\" - Produces "
\' - Produces '
\\ - Produces \
\x<2-digit hex number> - Same as chr(0x<2-digit hex number>)
\0 - Produces a null char
\1 - Produces a control char (same as chr(1))
\u<4-digit hex number> - Produces an UTF-8 character
\r - Produces a newline on old OSX
\n - Produces a newline on Linux/newer OSX/Windows (when writting a file without b)
\t - Produces a tab
\<number> or \0<number> - Same as \x, but the numbers are in octal (e.g.: "\75" and "\075" produce =)
... (some more that I probably forgot) ...
\<anything> - Produces <anything>
Read more about this on https://php.net/manual/en/language.types.string.php

Answer 2

Depending on the platrofm you're using, "/\\*{2,}/" may actually be a representation of a /\*{2,'}/ string - this is because languages like Java treat \ as an escape character, so to actually put that character within regex, you need to escape the character in regex string.
So, we have /\*{2'}/ regex. \*' matches the star character, and{2,}` means at least two times. Your regex will match any two or more consecutive star characters.

Answer 3

Is it a string literal written in a program and if so which one? The double backslash may be to escape the escape char so that this regex matches at least 2 * star characters.
In JavaScript for example you need to escape the \ so that your string literal can express it as data before you transform it into a regular expression when using the RegExp constructor. Why do regex constructors need to be double escaped?

Answer 4

For PHP what you have with that regex is to repeat literally a * 2 or more times. You can easily see with with below diagram:
But when you have to code it in PHP you have to escape the backslash (with a backslash) to use it in string. For instance:
$re = "/\\*{2,}/";
$str = "...";
preg_match($re, $str, $matches);