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Author: iliakan

10-regular-expressions-javascript/01-regexp-introduction/article.md

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+# Patterns and flags
+
+Regular expressions is a powerful way of searching and replacing inside a string.
+
+In Javascript regular expressions are implemented using objects of a built-in `RegExp` class and integrated with strings.
+
+Please note that regular expressions vary between programming languages. In this tutorial we concentrate on Javascript. Of course there's a lot in common, but they are a somewhat different in Perl, Ruby, PHP etc.
+
+[cut]
+
+## Regular expressions
+
+A regular expression (also "regexp", or just "reg") consists of a *pattern* and optional *flags*.
+
+There are two syntaxes to create a regular expression object.
+
+The long syntax:
+
+```js
+regexp = new RegExp("pattern", "flags");
+```
+
+...And the short one, using slashes `"/"`:
+
+```js
+regexp = /pattern/; // no flags флагов
+regexp = /pattern/gmi; // with flags g,m and i (to be covered soon)
+```
+
+Slashes `"/"` tell Javascript that we are creating a regular expression. They play the same role as quotes for strings.
+
+## Usage
+
+To search inside a string, we can use method [search](mdn:js/String/search).
+
+Here's an example:
+
+```js run
+let str = "I love Javascript!"; // will search here
+
+let regexp = /love/;
+alert( str.search(regexp) ); // 2
+```
+
+The `str.search` method looks for the pattern `pattern:/love/` and returns the position inside the string. As we might guess, `pattern:/love/` is the simplest possible pattern. What it does is a simple substring search.
+
+The code above is the same as:
+
+```js run
+let str = "I love Javascript!"; // will search here
+
+let substr = 'love';
+alert( str.search(substr) ); // 2
+```
+
+So searching for `pattern:/love/` is the same as searching for `"love"`.
+
+But that's only for now. Soon we'll create more complex regular expressions with much searching more power.
+
+```smart header="Colors"
+From here on the color scheme is:
+
+- regexp -- `pattern:red`
+- string (where we search) -- `subject:blue`
+- result -- `match:green`
+```
+
+
+````smart header="When to use `new RegExp`?"
+Normally we use the short syntax `/.../`. But it does not allow any variables insertions, so we must know the exact regexp at the time of writing the code.
+
+From the other hand, `new RegExp` allows to construct a pattern dynamically from a string.
+
+So we can figure out what we need to search and create `new RegExp` from it:
+
+```js run
+let search = prompt("What you want to search?", "love");
+let regexp = new RegExp(search);
+
+// find whatever the user wants
+alert( "I love Javascript".search(regexp));
+```
+````
+
+
+## Flags
+
+Regular expressions may have flags that affect the search.
+
+There are only 5 of them in Javascript:
+
+`i`
+: With this flag the search is case-insensitive: no difference between `А` and `а` (see the example below).
+
+`g`
+: With this flag the search looks for all matches, without it -- only the first one (we'll see uses in the next chapter).
+
+`m`
+: Multiline mode (will cover in [todo]).
+
+`u`
+: Enables full unicode support. The flag enables correct processing of surrogate pairs. More about that in the chapter <info:regexp-unicode>.
+
+`y`
+: Sticky mode (covered in [todo])
+
+
+
+## The "i" flag
+
+The simplest flag is `i`.
+
+An example with it:
+
+```js run
+let str = "I love Javascript!";
+
+alert( str.search(/LOVE/) ); // -1 (not found)
+alert( str.search(/LOVE/i) ); // 2
+```
+
+1. The first search returns `-1` (not found), because the search is case-sensitive by default.
+2. With the flag `pattern:/LOVE/i` the search found `match:love` at position 2.
+
+So the `i` flag already makes regular expressions more powerful than a simple substring search. But there's so much more. We'll cover other flags and features in the next chapters.
+
+
+## Summary
+
+- A regular expression consists of a pattern and optional flags: `g`, `i`, `m`, `u`, `y`.
+- Without flags and special symbols that we'll study later, the search by a regexp is the same as a  substring search.
+- The method `str.search(regexp)` returns the index where the match is found or `-1` if there's no match.