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Regular Expression (RegEx)

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While filling online forms, haven't you come across errors like "Please enter valid email address" or "Please enter valid phone number".

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Annoying as they may be, there's a lot of black magic that the computer does before it determines that, the details you've entered are incorrect.

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Can you think out, what is that black magic? If you are familiar with algorithms, then you will say that we can write an algorithm for the same.

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Yes, we can write an algorithm to verify different things, but we have a standard tool designed for similar kinds of purposes.

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It is Regular Expression. We call it RegEx for short. RegEx makes our work a lot easier. Let's see some basic examples where RegEx becomes handy.

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Suppose, you are in search of an averge price of a particular product on amazon. The following regular expression will find you any price(ex. $12, $75.50) on the webpage: \$([0-9]+)\.([0-9]+). In the Image below, yellowish part shows the matched prices.

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enter image description here

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Quite interesting!

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Let's look at another example. You have a long list of documents with different kinds of extensions. You are particularly looking for data files having .dat extension.

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^.*\.dat$ is a regular expression which represents a set of string ending with .dat. Regular expression is a standardized way to encode such patterns.

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Below in the image, you can see that all three files having .dat extension are extracted from the list of five files.

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enter image description here

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Well. What does the name Regular Expression(RegEx) represent? Regular Expression represents the sequence of characters that defines a regular search pattern.

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RegEx is a standardized tool to do the following works:

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  1. Find and verify patterns in a string.
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  2. Extract particular data present in the text.
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  3. Replace, split and rearrange particular parts of a string.
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We are going to look at all the three things above.

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Let's begin the journey with RegEx!

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Note:

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  1. Alpha-numeric character belongs to anyone of the $0-9,A-Z,a-z$ ranges.
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  2. String is a sequence of characters and substring is a contiguous part of a string.
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In this article series, we are going to show all examples using live interactive playground, so that you can play with regex. You can activate playground by just clicking on it.

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Simple Alpha-numeric character matching

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Simple matching of a specific word can be done as following:

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As you can see it matches "Reg" in the text. Similarly, what will be the match for "Ex" in the same text above?

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Do you notice anything? It is a case sensitive.

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Implementation

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Most of the programming languages have libraries for RegEx. They have almost similar kind of syntax. Here, we will see how to implement it in Javascript.

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Below is a basic code in Javascript, showing how to implement regex. The patterns are written in /_____/g. Where g is a modifier, which is used to find all matches rather than stopping at the first match.

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The function exec returns null, if there is no match and match data otherwise.

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var text_to_search_in = "RegEx stands for Regular Expression!";
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-var pattern = /Reg/g;
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-// This will print all the data of matches across the whole string
-while(result = pattern.exec(text_to_search_in)) {
-    console.log(result);
-}
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- The output will be: -
[
-  'Reg',
-  index: 0,
-  input: 'RegEx stands for Regular Expression!',
-  groups: undefined
-]
-[
-  'Reg',
-  index: 17,
-  input: 'RegEx stands for Regular Expression!',
-  groups: undefined
-]
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Note: Groups in the above output is a RegEx concept.

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