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  • Appendix 1 – Regular Expression commands

Appendix 1 – Regular Expression commands

A regex is a text string that describes a pattern that a regex engine uses in order to find text (or positions) in a body of text, typically for the purposes of validating, finding, replacing or splitting.
You can think of regular expressions as most advanced wildcards. But you can do much more with regular expressions.
You could use the regular expression  \b [ A-Z0-9._%+- ] +@[A-Z0-9.-]+\.[A-Z]{2,}\b to search for an email address.  Any  email address, to be exact.  
Regular Expressions Quick Start
This quick start gets you up to speed quickly with regular expressions. Obviously, this brief introduction cannot explain everything there is to know about regular expressions. For detailed information, consult the regular expressions tutorial. Each topic in the quick start corresponds with a topic in the tutorial, so you can easily go back and forth between the two.
Many applications and programming languages have their own implementation of regular expressions, often with slight and sometimes with significant differences from other implementations. When two applications use a different implementation of regular expressions, we say that they use different "regular expression flavors". This quick start explains the syntax supported by the most popular regular expression flavors.
Text Patterns and Matches
A regular expression, or regex for short, is a pattern describing a certain amount of text. On this website, regular expressions are highlighted in red as regex. This is actually a perfectly valid regex. It is the most basic pattern, simply matching the literal text regex. Matches are highlighted in blue on this site. We use the term "string" to indicate the text that the regular expression is applied to. Strings are highlighted in green.
Characters with special meanings in regular expressions are highlighted in various different colors. The regex (?x)([Rr]egexp?)\? shows meta tokens in purple, grouping in green, character classes in orange, quantifiers and other special tokens in blue, and escaped characters in gray.
Literal Characters
The most basic regular expression consists of a single literal character, such as a. It matches the first occurrence of that character in the string. If the string is Jack is a boy, it matches the a after the J.
This regex can match the second a too. It only does so when you tell the regex engine to start searching through the string after the first match. In a text editor, you can do so by using its "Find Next" or "Search Forward" function. In a programming language, there is usually a separate function that you can call to continue searching through the string after the previous match.
Twelve characters have special meanings in regular expressions: the backslash \, the caret ^, the dollar sign $, the period or dot ., the vertical bar or pipe symbol |, the question mark ?, the asterisk or star *, the plus sign +, the opening parenthesis (, the closing parenthesis ), the opening square bracket [, and the opening curly brace {. These special characters are often called "metacharacters". Most of them are errors when used alone.
If you want to use any of these characters as a literal in a regex, you need to escape them with a backslash. If you want to match 1+1=2, the correct regex is 1\+1=2. Otherwise, the plus sign has a special meaning.
Character Classes or Character Sets
A "character class" matches only one out of several characters. To match an a or an e, use [ae]. You could use this in gr[ae]y to match either gray or grey. A character class matches only a single character. gr[ae]y does not match graay, graey or any such thing. The order of the characters inside a character class does not matter.
You can use a hyphen inside a character class to specify a range of characters. [0-9] matches a single digit between 0 and 9. You can use more than one range. [0-9a-fA-F] matches a single hexadecimal digit, case insensitively. You can combine ranges and single characters. [0-9a-fxA-FX] matches a hexadecimal digit or the letter X.
Typing a caret after the opening square bracket negates the character class. The result is that the character class matches any character that is not in the character class. q[^x] matches qu in question. It does not match Iraq since there is no character after the q for the negated character class to match.
Shorthand Character Classes
\d matches a single character that is a digit, \w matches a "word character" (alphanumeric characters plus underscore), and \s matches a whitespace character (includes tabs and line breaks). The actual characters matched by the shorthands depends on the software you're using. In modern applications, they include non-English letters and numbers.
The Dot Matches (Almost) Any Character
The dot matches a single character, except line break characters. Most applications have a "dot matches all" or "single line" mode that makes the dot match any single character, including line breaks.
gr.y matches gray, grey, gr%y, etc. Use the dot sparingly. Often, a character class or negated character class is faster and more precise.
Alternation is the regular expression equivalent of "or". cat|dog matches cat in About cats and dogs. If the regex is applied again, it matches dog. You can add as many alternatives as you want: cat|dog|mouse|fish.
Alternation has the lowest precedence of all regex operators. cat|dog food matches cat or dog food. To create a regex that matches cat food or dog food, you need to group the alternatives: (cat|dog) food.
The question mark makes the preceding token in the regular expression optional. colou?r matches colour or color.
Grouping and Capturing
Place parentheses around multiple tokens to group them together. You can then apply a quantifier to the group. E.g. Set(Value)? matches Set or SetValue.
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