jhdk707 / regex-tutorial

Dr. Regex: How I Learned to Stop Worrying and Love Regular Expressions

A Regex (regular expression) is a sequence of characters that define a search pattern. Generally, these patterns are used by algorithms that use a string-searching method for "find" or "find and replace" operations on strings, as well as searching for input validation.

Summary

In this tutorial, we will be explaining the components of a regular expression, and how it works to create patterns for matching text. We will cover anchors, quantifiers, grouping contructs, bracket expressions, character classes, OR operator, flags, and character escapes.

We will be examining a Regex dealing with matching an email. Here is an example of a string that would do that- /^([a-z0-9_\.-]+)@([\da-z\.-]+)\.([a-z\.]{2,6})$/ and below we will discuss the components.

Table of Contents

• Anchors
• Quantifiers
• Grouping Constructs
• Bracket Expressions
• Character Classes
• The OR Operator
• Flags
• Character Escapes

Regex Components

Anchors

Anchors are used to specify the position of a pattern within the text being matched. The two most common anchors used in regular expressions are ^ (the Caret symbol) and $ (the Dollar sign). The Caret symbol is used to match the beginning of a line, and the Dollar sign is used to match the end of a line.

You can also use \b which matches a word boundary position between a word character and a non-word character or position (start/ end of string) or you can use \B which will match any position that is not a word boundary - it only matches positions, not a character.

Quantifiers

Quantifies specify how many time a preceding element should occur in the text being matched. There are many quantifiers that you can use:

• * The asterisk is used to match zero or more occurrences of the preceding element
• + The plus sign is used to match one of more occurrences of the preceding element
• ? The question mark is used to match zero or one occurrences of the preceding element
• {n} Curly braces are used to specify an exact number of the occurrences of the preceding element
• {n,} Curly braces with a comma are used to specify a minumum number of occurrences of a preceding element
• {n,m} Curly braces with a comma between two values is used to specify a range of occurrences of a preceding element

Grouping Constructs

• Capturing group () - Groups together elements and also captures and remembers matched text for later use, referenced or retrieved using backreferences
• Non Capturing group (?: ) - Similar to capturing groups as in they group together elements, but they do not capture or remember the matched text. Useful when needing to apply operations to a group of elements but do not need to reference them later.
• Lookahead and Lookbehind assertions allow you to assert if certain characters or patterns are present of not present ahead of or behind the current position without consuming characters in the match. The symbols here are listed below:
  • Lookahead Positive (?= )
  • Lookahead Negative (?! )
  • Lookbehind Positive (?<= )
  • Lookbehind Negative (?<! )

Bracket Expressions

In this example ([a-z0-9_\.-]+) is a bracket expression that matches one or more lower case letters, digits, underscores, dots or hyphens in the username part of an email address. This expression ([a-z\.]{2,6}) matches exactly 2 to 6 lowercase letters of dots, which represent the top-level domain (TLD) part of an email address

Character Classes

Character Classes are enclosed in [] in regex and define a set of characters that can match a single character in the input text.

• @ Matches the "@" symbol, and \. matches a dot character
• [a-z0-9_\.-] Matches any lowercase letter a-z, digits 0-9, underscore, dot or hyphen
• [\da-z\.-] Matches any digit, lowercase letter, dot or hyphen
• [a-z\.] Matches any lowercase letter or dot

The OR Operator

The OR operator in regular expressions is denoted by the vertical bar | and allows for matching either one pattern or another. In the given regular expression, the OR operator is used in the character class ([a-z\.]{2,6}) to allow for matching either lowercase letters (a to z) or dots (.), and the quantifier {2,6} specifies that there must be between 2 to 6 occurrences of the preceding pattern (i.e., lowercase letters or dots). This allows for matching top-level domain (TLD) names that consist of 2 to 6 characters, such as .com, .org, .edu, etc.

Flags

This particular example does not contain any flags, however flags are optional parameters that modify the behavior of the regex pattern matching. They are typically added after the closing delimiter of the regular expression and are used to specify options such as case-insensitivity, global matching, multi-line mode, etc. Examples of common flags include i for case-insensitive matching, g for global matching, and m for multi-line mode.

Character Escapes

Character escapes are special sequences of characters that have a specific meaning in regular expressions. In this regex, the following character escapes are used:

• \d - Represents a digit character (0-9).
• . - Represents a period character. However, it is not used as a character escape in this regex, but as a literal period character, as it is not preceded by a backslash

Author

This Regex Gist was written by Jesse Hudak for the UC Berkeley EDX Full-Stack Cohort on 4/13/23. You may view my other work here