# Regular Expressions, Searching, Replacing, Building

## What is a Regular Expression

* A regular expression (**`regex`**) is a pattern that matches a set of strings, consisting of operators, constructs literal characters and meta-characters.

> 📌 **grep** command supports tree regex syntaxes. Check some basics usage [here](https://linuxize.com/post/regular-expressions-in-grep/).

* Regular expressions are powerful, used in areas like search engines, programming languages and text processing applications.

### Examples

* Check [how to find or validate an Email Address](https://www.regular-expressions.info/email.html) - [RFC 5322](https://www.ietf.org/rfc/rfc5322.txt) Official Standard
* More examples here [emailregex.com](https://emailregex.com/)

```bash
# Find all email addresses in a file using grep
grep -E -o "\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,6}\b" filename.txt

# Python
r"(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)"

# Javascript
/^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/

#MySQL
SELECT * FROM `users` WHERE `email` NOT REGEXP '^[A-Z0-9._%-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$';
```

## Searching with RegEx

* There a 4 primary components into a regular expression:
  * character classes
  * quantifiers and alternation
  * anchors
  * roots and anchors

### [Date Search](https://regexr.com/66ogg)

```javascript
^[A-Z][a-z]{2,}\s+[0-3]?[1-9],\s+[12]?[0-9]{0,3}$
/*
^ = ANCHOR - match the beginning of the string or line (with the multiline flag m)
[A-Z][a-z] = CHARACTER CLASSES - range matching upper/lower case letters
{2,} = QUANTIFIER - match 2 or more of the preceding token
\s = whitespace
+ = QUANTIFIER - match 1 or more of the preceding whitespace
* = QUANTIFIER - match 0 or more of the preceding token
? = QUANTIFIER - optional, match 0 or 1 of the preceding token
| = QUANTIFIER - alternation, match 1 token or another
$ = ANCHOR - match the end of the string or line (with the multiline flag m)
*/
```

![](/files/gFQiTYbR0BvPL3imLApT)

### [Number >= 42 Search](https://regexr.com/66ogj)

```javascript
^4[2-9]|[5-9]\d|[1-9]\d{2,}$
/*
\d = match any digit character (0-9) = [0-9]
*/
```

![](/files/FZ0eItGpYnozEsRaT64s)

![](/files/uqjrdIivnjhyYikYL6ha)

```bash
grep -E "^4[2-9]|[5-9]\d|[1-9]\d{2,}$" numbers.txt
	42
# grep (posix) doesn't have the \d shortcut for digits

grep -E "^4[2-9]|[5-9][1-9]|[1-9][1-9]{3,}$" numbers.txt
    42
    55
    95
```

![](/files/EGXUqA5YdMlqM1Cx6878)

### [Dollar $ Search](https://regexr.com/66ogm)

```javascript
^\$?\s*[1-9][0-9]{0,2}(,?[0-9]{3})*(\.[0-9]{2})?$

/*
\$ = ESCAPED CHAR - match a "$" character
*/
```

![](/files/YVeCzFLJ4r0abymx0Ydj)

## Replacing with RegEx

* Replacing text with regular expressions varies between implementations.
  * Examples with the regexr site tool:

![](/files/XFgJWB2yIJbOaMtbfGR7)

![](/files/hlK0tU3EH7z4J7hVvdig)

## Tips on Building RegEx

* Regular expressions are very powerful but are not appropriate for every problem.
* Regular expressions are greedy (**they'll match as much as they can**).
  * Add and **`?`** after **`*`** or **`+`** too make the match lazy (match the minimum possible)
* **Don't write an entire regex all at once.**
  * Build a piece - test it - repeat
  * Use multiple, simpler, smaller expressions
* Test with **valid** and **invalid** data - ensure the regex matches only what you want it to match.
* Add comments using **`x`** modifier.

***


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://blog.syselement.com/tcm/courses/linux-101/9-regex/regex-use.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.