Regular Expressions Overview (RegEx)

Overview

Taken from Wikipedia, the free encyclopedia

A regular expression (shortened as regex or regexp; sometimes referred to as rational expression) is a sequence of characters that specifies a search pattern in text. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings, or for input validation.

Basic Syntax

In this example RegEx:

/([A-Z])\w+/g

There's a lot of dense syntax here, let's list it out:

• The 1st / starts the RegEx
• The last / ends the RegEx
• The final characters, like g, are flags
  • g means a global flag
    • It will find the pattern everywhere, not just the first occurrence
  • Other flags exist that will be outlined later
    • These other flags often depend on the software using it
• The () parentheses define a capture group
  • They get used to group several matching tokens together
  • In this case they enclose...
• [], a character set, which matches any character in the set within
  • Inside these brackets characters to match can be defined
  • This includes the above example's...
• A-Z range, which matches a character in range A to Z separated by -
  • A to Z has an ASCII & UTF character code 65 to 90 in decimal, in order
  • Ranges can be specified for characters and numbers
  • Case sensitive so a separate range is needed for lower case
• \W is a special token representing a word
  • Matches any word character (alphanumeric & underscore)
  • There's several other special tokens like this that will get detailed later
• + is a quantifier to match 1 or more of the preceeding tokens (words)

Further Exploration

This example can be explored in detail because it is the default Regex pattern in RegExr.com. There there's an interactive playground of text and RegEx patterns. It gives a more visually easy to discern explanation of this pattern. The web app can also be used to test or practice different patterns and look up reference material on the subject of regular expressions.

Character Classes

Here are some character classes to help in making RegEx selections. These will help with defining what characters get matched.

Note: Any row above with the * footnote indicate spaces `` are being matched as well. They don't get picked up with bold markings.

Anchors

The first two rows above show anchors relating to the end and beginnings of strings. The second two are anchors related to the boundraries of words. Boundraries are any characters that end a word like whitespace or punctuation.

Qualifiers and Alteration

In the first two qunatity matchers + & * you can see how the + won't select the b alone, it needs a \w or word character to follow. The * will select all words because they all start with b and because it selects 0 or more it selects the lone b as well.

The {1,3} curly braces sets the number of matches of a character we want. In the example it wants a b followed by any word character from two to three occurrences.

The ? quantifier makes the preceeding character optional. By specifying colou?r you can check for the Brittish & American pronounciation of color. The u becomes optional because it is followed by ?. It also can be used as a lazy alternation where

Regular Expression Patterns

Let's see how we apply these patterns.

References

Web Links

• RegExr: A Regular Expressions Playground & Teaching Tool

Note Links

• Regular expression (from Wikipedia, the free encyclopedia)

Referenced By

• PCDE Course Overview
• Structured Query Language (SQL)