Regular Expressions - Overview

regex.md

Regex:

1. Introduction
2. Modes
3. Literal characters
4. Metacharacters
5. Backreferences
6. Special Characters
7. Useful Expressions

Introduction

Regular expressions are symbols representing a text pattern. They are used for matching, searching and replacing text.
The goal in regular expressions is to match both what you want and only what you want!

Modes

• Standard - /re/
• Global - /re/g
• Case-insensitive - /re/i
• Multiline Anchors - /re/m
• Dot-matches-all - /re/s

Modes are defined after the last / of the regular expression, and could be used together. For example, using both global and case insensitive modes: /re/gi

Literal characters

/car/ matches "car"
/car/ matches the first three letters of "carnival"

• Case sensitive by default (best practice)
  For example: /car/ doesn't match anything in "Carnival"
• Standatd (non-global) matching - earliest (leftmost) match is always prefered.

Example:

word: "pazzazz"
/zz/ - will match pazzazz
/zz/g - will match pazzazz

Metacharacters

There are only few metacharacters to learn:
\ . * + - { } [ ] ^ & $ | ? ( ) : ! =

• . - Any character except newline
  Examples:

• /h.t/ - matches "hot" , "hat" , "hit" but not "heat"
• /.a.a.a/ - matches "banana" , "#aga!a" , " a asa"

Notice for common mistake:
/9.00/ - matches "9.00", "9500" and "9-00"

• \ - Escape the next metacharacters
  Note that literal characters shouldn't be escaped
  Examples:

• /9\.00/ - matches "9.00" but not "9500" or "9-00"
• /\/home\/usr\/doc\.txt/ - matches "/home/usr/doc.txt"

• [,] - Defining a character set (begin and end), but only one
  Order of characters does not matter
  Note: Metacharacters shouldn't be escaped inside a character set - they are already escaped (Except ], -, ^, \) Examples:

  • /gr[ea]y/ - matches both "grey" and "gray"
  • /gr[ea]t/ - doesn't match "great"
  • /h[abc.xyz]t/ - matches "hat" and "h.t" - the . is already escaped.
  • /var[[(][0-9][)\]]/ - matches "var(3)" and "var(4)"
  • /file[0\-\\_]1/ - matches "file01", "file-1", "file\1" and "file_1"

  Shorten character set:

  • \d - all digits (same as [0-9])
  • \w - work character (same as [a-zA-Z0-9_])
  • \s - whitespace (same as [ \t\r\n])
  • \D - not digits (same as [^0-9])
  • \W - not work character (same as [^a-zA-Z0-9_])
  • \S - not whitespace (same as [^ \t\r\n])

• - - Range of characters - represents all characters between two characters
  Only inside a character set
  Examples:

  • /[0-9]/ - matches for any digit
  • /[A-Za-z]/ - matches for all letters
  • /[a-ek-ou-y]/ - any letter in the specified range

Caution:
/[50-99]/ - is not all numbers from 50 to 99

• ^ - Negate a character set - adding it as the first of character set
  Still represents only one character
  Examples:
  • /see[^mn]/ - matches "seek" and "sees" but not "seem" or "seen"

Caution:
/see[^mn]/ - matches "see " but not "see"

• * - Preceding item zero or more times
  Examples:

  • /apples*/ - matches "apple", "apples" and "applessss"
  • /\d\d\d\d*/ - matches numbers with three digits or more

• + - Preceding item one or more times
  Examples:

  • /apples+/ - matches "apples" and "applessss", but not "apple"
  • /<[^>]+>/ - matches any HTML tag

• ? - Preceding item zero or one time
  Note that literal characters shouldn't be escaped
  Examples:

  • /apples*/ - matches "apple", "apples" but not "applessss"
  • /colou?r/ - matches "color" and "colour"

• {, } - Starting and ending quantified repetition of preceding item
  Getting {min,max} - positive numbers. Min must always be included (can be zero). Max is optional.
  Examples:

  • /\d{4,8}/ - matches numbers with four or eight digits
  • /\d{4}/ - matches numbers exactly four digits
  • /\d{4,}/ - matches numbers with four or more digits (max is infinite)

• (, ) - Grouping metacharacters
  Makes the expressions easier to read. Cannot be used inside character set.
  Examples:

  • /(abc)+/ - matches "abc" and "abcabcabc"
  • /(in)?dependent/ - matches "independent" and "dependent"
  • /run(s)?/ - is the same as /runs?/

• | - Match previous or next expression
  Examples:

  • /apple|orange/ - matches "apple" and "orange"
  • /w(ei|ie)rd/ - matches "weird" and "wierd"
  • /(AA|BB|CC){6}/ - matches "AABBAACCAABB" and more..
  • /(\d\d|[A-Z][A-Z]){3}/ - matches "112233", "AA66ZZ", "11AA44" and more..

• Anchors Metacharacters:
  Anchors refers to a position, not an actual character. They are zero-width.

  ^: Start of string / line. (Not the same as at start of a character set)
  $: End of string / line
  Examples:

  • /^apple/ - matches "apple" only if it's on a beginning of a string/line
  • /apple$/ - matches "apple" only if it's on a end of a string/line

Backreferences:

Stores the matched portion in parentheses.
/a(p{2}l)l/ matches "apple" and stores "ppl". It is done automatically by default.
Refer to first backreference with \1.
\1 through \9 - backreferences for positions 1 to 9.
Usage:

• Can be used in the same expression as the group.
• Can be accessed after the match is complete (programming language needed).
  Examples:

• /(apples) to \1/ - matches "apples to apples"
• /(ab)(cd)(ed)\3\2\1/ - matches "abcdefefcdab"
• /<(i|em)>.+?</\1>/ - matches "Hello" and "Hello"

Special Characters:

• Spaces - space is a regular character
• Tabs - tabs are matchable by \t
• Line - \r, \n, \r\n
  ** depends on your file mode
• Non-printable characters:
  • bell \a
  • escape \e

Useful expressions:

• Names:
  • /^\w+/ - Not that good solution
  • /^[A-Z][a-z.']+ [A-Z][a-z.']+/ - Matches first name and last
• Email Adresses:
  • /^[\w.\-]+@[\w.\-]+\.[A-Za-z]{2,3}$/ - Matches email
• URLs:
  • /^(http|https):\/\/[\w.\-]+(\.[\w\-]+)+[/#?]?.*$/
• IPs:
  • /^(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$/m - It is long, but assures that we won't get higher than 255 for each number.
• HTML tags:
  • /<([^>]+)>(.*?)</\1>/