Why does GREP expression need two back-slashes?

[Schmaltzkopf]

Hi, there.

Curious question:

I have the following code:

   psYSbody.nestedGrepStyles.add({

        appliedCharacterStyle: csYSimporedPsukimMekor,

        grepExpression: "\\(.+?\\)"});

Why does this only work if I have double-backslashes before "()" and "[]"?

If I only have a single backslash it will totally ignore the backslash.

Anyone could explain this quirk?

[Trevor: · Accepted solution]

Hi

The "/" character is an escape character. For example if you want to alert "the "big" cat" with the word cat in quotes one way of doing it is alert("the \"big\" cat");

Because you are providing a string you need to escape the escape

Try the following 2 examples

alert("\(") // => (
alert("\\(") // => \(

What you want is the second example as the "correct" grep is \( and not (

Got it?

Trevor

[Schmaltzkopf]

Thank you Trevor.

I'm used to the UI GREP codes, so I appreciate your short and sweet explanation that JS strings are different

[Marc Autret]

Hi,

Another way to explain the double-escape trick is to consider those two distinct levels:

1. At the GREP (or regular expression) level, parentheses ( and ) are special symbols that control capturing groups.

So if you want to capture a left parenthesis as itself you need to use an escape sequence.

In GREP, the escape character is \ (backslash) so that the full escape sequence is \( (backslash + parenthesis.)

2. At the JavaScript level (which is the language you are using to send the above GREP command), the backslash \ is a special symbol when involved in a literal string. Indeed, that symbol is used to form escape sequences!

So if you want to provide the backslash as itself in a literal string, you need the following escape sequence, "\\" (backslash + backslash), which in fact represents a single backslash character.

Finally, in order to encode in JavaScript the full GREP pattern \( —that is, a backslash and a left parenthesis—you'll have to use the literal string "\\(".

[The reasoning is the same for square brackets.]

Hope that helps.

@+

Marc