# Intro to Regular Expressions
## Introduction
Being a Linux user and not coming across Regular Expressions or regex is next to impossible. I kept seeing the cryptic little set of characters everywhere and the people who could use it looked like ninjas to me. They are everywhere text editor's, JavaScript, Python, JAVA , Bash......and the list goes on.
{{< figure src="https://imgs.xkcd.com/comics/regular_expressions.png" title="XKCD Regex Comic" >}}
## What Exactly is a Regular Expression?
A regular expression is a string containing a combination of normal characters and special meta-characters or meta-sequences. The normal characters match themselves. Meta-characters and meta-sequences are characters or sequences of characters that represent ideas such as quantity, locations,or types of characters.
### Literals (literally match this)
Something that we normally do while searching for text matches.
Character by character matching is done.
Sample Text:
`hello world`
With the below regex you would match literals since there are no special or meta-characters.
`hello`
Matches the following characters in source string.
`hello` world
## Meta-characters
### Single Character (what to match?):
| Option | Description |
| :--------------: | :-----------------------------------------: |
| . | anything |
| \d | digit in 0123456789 |
| \D | non-digit |
| \w | "word" letter digits and \_ |
| \W | non-word |
| (a single space) | space |
| \t | tab |
| \r | return |
| \n | newline |
| \s | whitespace ( (a single space), \t, \r, \n ) |
| \S | non-whitespace |
### Quantifiers (How many occurence to match?)
| Option | Description |
| :----: | :----------------------------------------: |
| X\* | Zero or more occurence of X |
| X+ | One or more occurence of X |
| X\* | Exactly m instances of X |
| X{m} | One or more instances occurence of X |
| X{m,} | Atleast m instances of X |
| X{m,n} | between m and n instances (inclusive) of X |
{{< admonition note >}}
Quantifiers characters comes after what character we want to
repeat not before it.
{{< /admonition >}}
- `+T` is invalid
- `T+` means one or more occurrence of the character T
- `u?` means check for 0 or 1 occurrence of u in the string
- `?u` is invalid
#### Position (where?)
| Option | Description |
| :----: | :------------------------------------------------------: |
| \b | Word Boundaires as defined as any edge between \w and \W |
| \B | non-word-boundaries |
| ^ | the beginning of the line |
| \$ | the end of the line |
## Practising the what, How many and where of regular expressions
Now that we are done with the absolute basics of regular expressions. Whenever writing a regular expression ask yourself what are the "What", "How many", and "Where" of the regular expression
The best way to learn regular expression is through Trial and Error.
{{< figure src="/2019-07-02/trialerror.jpg" title="Regex by Trial and Error" >}}
To follow along open up a test regex matching tool in your text editor or [Online regex matching tools](https://regexr.com/)
Scenario 1:
Matching the word cat
- **what to match** : 'cat'
- **How many** : None,Since we are doing literal matching with no repetitions (we want to match cat not catcat).
- **Where** : 'cat' that do not belong inside any word and are whole words.
regex : `\bcat\b`
Specifies a regular expressions that matches the literal cat that comes between world boundaries (word boundaries mark the start or end of a word), without the world boundaries the regex will match all occurrence of 'cat' in the text. Always remember there is a cat in catastrophe. And the importance of word boundaries.
Scenario 2:
Match all occurrences of 10 digit phone numbers
- **What to match** : All numbers \d (for digit match)
- **How many** : 10 occurrences.
- **Where** : not needed
## Character Classes and Alternations
#### Character classes (anything in between [ ])
Think of them as the OR operation in programming. We specify in the
character class braces any alternations of character that need to be
matched.
- `[ab]` means matches for `a` or`b`
- `f[au]n` matches for `fan` and `fun`
_Note: if you specify single character meta-character inside [ ] it
loses its special characteristics_
But there are characters that have special meaning inside [] braces in
regular expressions they are
`^` - Anything except what is specified. `[^ab]` means match anything
except a or b
`-` - If it is the first character inside the [ ] it is treated as a
literal - but if in between different characters or digits it specifies
a range [a-z] means lowercase characters from a to z and [0-10] means
numbers from 0 to 10.
#### Alternations ( anything in between ( | ) )
What if we needed to give an or option for a set of character like match
a 'net' or a 'com'
we can use (net|com) for multi-character option matching.
Example:
`www.facebook.(com|net)` matches `www.facebook.com` and `www.facebook.net`
## Advanced Regular Expressions with Groups and Back References
#### Groups
Regular expressions can be divided into parts that can be later used for
replace and other operations.
`$0` is the whole regular expression
`$1` is the first part .
`$2` is the second part and so on from the left to right.
In the following example we do a search and replace of Last-name ,
First-name to First-Name Last-name
{{< figure src="/2019-07-02/regex.png" title="Groups in regular expression" >}}
### Back References
It is a way of specifying a previous group of regular expression later
in the same regular expression using \\GroupNo .
Imagine a scenario where we are searching for occurrence of the same
word one after.
`hello hello` `ha ha` `kill kill`
we make use of the regex `(\w+)\s\1` which uses the \\1 to specify the
same pattern as the first group
## Reading Regular Expressions
`^\w\w\w\S$` - (\^)begin with any word character(\\w) and then two more
word characters then anything except a space.
`^T+\w\w\d$` - (\^)begin with one or more (+) occurrence of T followed
by two (\\w)word characters then a (\\d)digit and then the end.