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+---
+title:  "Learning Regex"
+description: "Utilize pattern matching to comb through texts"
+author: Shriansh Chari
+date:   2021-12-25
+categories: technical
+---
+
+### What is Regex?
+
+Regex (short for **regular expressions**) are a sequence of characters that represent a search pattern to search in a text. At first, regex can look intimidating with its obtuse syntax, but once you learn to break them down, they are a valuable tool for working with the terminal or for verifying user input.
+
+### Basic Regex Patterns
+
+| Pattern      | Will match                                                   |
+| ------------ | ------------------------------------------------------------ |
+| `.`          | Any **single** character                                     |
+| `*`          | Preceding range will be matched 0 or more times.             |
+| `+`          | Preceding range will be matched 1 or more times.             |
+| `{N}`        | Preceding range will be matched exactly `N` consecutive times. |
+| `[<range>]`  | Square brackets define a character set, will match any character included in the set. Examples: `[A-Za-z0-9]` matches any uppercase or lowercase letter, and any number, and `[aeiou]` matches exactly the characters `a`, `e`, `i`, `o`, and `u`. |
+| `[^<range>]` | The `^` symbol indicates that the range matches to the characters **not** in the set. Example: `[^aeiou]` matches any character except `a`, `e`, `i`, `o`, and `u`. |
+
+### Example Usage:
+
+Suppose you had a file (`numbers.txt`) with some phone numbers:
+
+```
+Phone numbers:
+09-912-12921
+011-011-0011
+430-129-1293
+238-12-12321
+721-1432-234
+431-123-0941
+```
+
+If you notice, some of these phone numbers aren't in the correct format. We can filter out the good phone numbers with the regular expression:
+
+`[0-9]{3}-[0-9]{3}-[0-9]{4}`
+
+where `[0-9]{3}` indicates 3 consecutive numbers, `[0-9]{4}` indicates 4 consecutive numbers, and the `-` indicates dashes in between each series of numbers.
+
+#### Executing Regex in the Terminal
+
+We can actually use the regular expression to match good phone numbers from the file. In the terminal, using the command format
+
+```bash
+egrep 'regex' file
+```
+
+we can execute the command
+
+```bash
+egrep '[0-9]{3}-[0-9]{3}-[0-9]{4}' numbers.txt
+```
+
+and it outputs the phone numbers that are in the correct format:
+
+```
+011-011-0011
+430-129-1293
+431-123-0941
+```
+
+### Practicing Regex
+
+Want to practice regular expressions? Go to these websites to practice making them yourself:
+
+- https://regexr.com/
+- https://regex101.com/
+