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9-regular-expressions/07-regexp-escaping/article.md

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+# Escaping, special characters
+
+As we've seen, a backslash `"\"` is used to denote character classes. So it's a special character in regexps (just like in a regular string).
+
+There are other special characters as well, that have special meaning in a regexp. They are used to do more powerful searches. Here's a full list of them: `pattern:[ \ ^ $ . | ? * + ( )`.
+
+Don't try to remember the list -- soon we'll deal with each of them separately and you'll know them by heart automatically.
+
+## Escaping
+
+Let's say we want to find a dot literally. Not "any character", but just a dot.
+
+To use a special character as a regular one, prepend it with a backslash: `pattern:\.`.
+
+That's also called "escaping a character".
+
+For example:
+```js run
+alert( "Chapter 5.1".match(/\d\.\d/) ); // 5.1 (match!)
+alert( "Chapter 511".match(/\d\.\d/) ); // null (looking for a real dot \.)
+```
+
+Parentheses are also special characters, so if we want them, we should use `pattern:\(`. The example below looks for a string `"g()"`:
+
+```js run
+alert( "function g()".match(/g\(\)/) ); // "g()"
+```
+
+If we're looking for a backslash `\`, it's a special character in both regular strings and regexps, so we should double it.
+
+```js run
+alert( "1\\2".match(/\\/) ); // '\'
+```
+
+## A slash
+
+A slash symbol `'/'` is not a special character, but in JavaScript it is used to open and close the regexp: `pattern:/...pattern.../`, so we should escape it too.
+
+Here's what a search for a slash `'/'` looks like:
+
+```js run
+alert( "/".match(/\//) ); // '/'
+```
+
+On the other hand, if we're not using `/.../`, but create a regexp using `new RegExp`, then we don't need to escape it:
+
+```js run
+alert( "/".match(new RegExp("/")) ); // '/'
+```                                                                                                                                                                                   
+
+## new RegExp
+
+If we are creating a regular expression with `new RegExp`, then we don't have to escape `/`, but need to do some other escaping.
+
+For instance, consider this:
+
+```js run
+let reg = new RegExp("\d\.\d");
+
+alert( "Chapter 5.1".match(reg) ); // null
+```
+
+The search worked with `pattern:/\d\.\d/`, but with `new RegExp("\d\.\d")` it doesn't work, why?
+
+The reason is that backslashes are "consumed" by a string. Remember, regular strings have their own special characters like `\n`, and a backslash is used for escaping.
+
+Please, take a look, what "\d\.\d" really is:
+
+```js run
+alert("\d\.\d"); // d.d
+```
+
+The quotes "consume" backslashes and interpret them, for instance:
+
+- `\n` -- becomes a newline character,
+- `\u1234` -- becomes the Unicode character with such code,
+- ...And when there's no special meaning: like `pattern:\d` or `\z`, then the backslash is simply removed.
+
+So the call to `new RegExp` gets a string without backslashes. That's why the search doesn't work!
+
+To fix it, we need to double backslashes, because quotes turn `\\` into `\`:
+
+```js run
+*!*
+let regStr = "\\d\\.\\d";
+*/!*
+alert(regStr); // \d\.\d (correct now)
+
+let reg = new RegExp(regStr);
+
+alert( "Chapter 5.1".match(reg) ); // 5.1
+```
+
+## Summary
+
+- To search special characters `pattern:[ \ ^ $ . | ? * + ( )` literally, we need to prepend them with `\` ("escape them").
+- We also need to escape `/` if we're inside `pattern:/.../` (but not inside `new RegExp`).
+- When passing a string `new RegExp`, we need to double backslashes `\\`, cause strings consume one of them.