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Merge pull request #2376 from vsemozhetbyt/patch-8

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Make solution in 9.14 safer
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Ilya Kantor 2020-12-13 20:21:27 +03:00 • committed by GitHub
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16
9-regular-expressions/14-regexp-lookahead-lookbehind/2-insert-after-head/solution.md
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@ -1,4 +1,4 @@
In order to insert after the `<body>` tag, we must first find it. We can use the regular expression pattern `pattern:<body.*>` for that.
In order to insert after the `<body>` tag, we must first find it. We can use the regular expression pattern `pattern:<body.*?>` for that.
In this task we don't need to modify the `<body>` tag. We only need to add the text after it.
@ -6,18 +6,18 @@ Here's how we can do it:
```js run
let str = '...<body style="...">...';
str = str.replace(/<body.*>/, '$&<h1>Hello</h1>');
str = str.replace(/<body.*?>/, '$&<h1>Hello</h1>');
alert(str); // ...<body style="..."><h1>Hello</h1>...
```
In the replacement string `$&` means the match itself, that is, the part of the source text that corresponds to `pattern:<body.*>`. It gets replaced by itself plus `<h1>Hello</h1>`.
In the replacement string `$&` means the match itself, that is, the part of the source text that corresponds to `pattern:<body.*?>`. It gets replaced by itself plus `<h1>Hello</h1>`.
An alternative is to use lookbehind:
```js run
let str = '...<body style="...">...';
str = str.replace(/(?<=<body.*>)/, `<h1>Hello</h1>`);
str = str.replace(/(?<=<body.*?>)/, `<h1>Hello</h1>`);
alert(str); // ...<body style="..."><h1>Hello</h1>...
```
@ -26,11 +26,11 @@ As you can see, there's only lookbehind part in this regexp.
It works like this:
- At every position in the text.
- Check if it's preceeded by `pattern:<body.*>`.
- Check if it's preceeded by `pattern:<body.*?>`.
- If it's so then we have the match.
The tag `pattern:<body.*>` won't be returned. The result of this regexp is literally an empty string, but it matches only at positions preceeded by `pattern:<body.*>`.
The tag `pattern:<body.*?>` won't be returned. The result of this regexp is literally an empty string, but it matches only at positions preceeded by `pattern:<body.*?>`.
So we replaces the "empty line", preceeded by `pattern:<body.*>`, with `<h1>Hello</h1>`. That's the insertion after `<body>`.
So we replaces the "empty line", preceeded by `pattern:<body.*?>`, with `<h1>Hello</h1>`. That's the insertion after `<body>`.
P.S. Regexp flags, such as `pattern:s` and `pattern:i` can also useful: `pattern:/<body.*>/si`. The `pattern:s` flag makes the dot `pattern:.` match a newline character, and `pattern:i` flag makes `pattern:<body>` also match `match:<BODY>` case-insensitively.
P.S. Regexp flags, such as `pattern:s` and `pattern:i` can also be useful: `pattern:/<body.*?>/si`. The `pattern:s` flag makes the dot `pattern:.` match a newline character, and `pattern:i` flag makes `pattern:<body>` also match `match:<BODY>` case-insensitively.