Introduction to Regular Expressions

Written by Barry Dysert (last updated February 22, 2021)

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Regular expressions are fairly common in computer programming. But they are also available to end-users, as well. For example, if you're dealing with PowerShell or the Findstr command, you can make use of them. And there are third-party utilities (such as Agent Ransack and EditPad) that support regular expressions. Since they are so useful in many different environments, I decided that a tip about them is in order.

A regular expression is a string, often consisting of special characters, that is designed to match a search pattern. For example, if we're using the Findstr command, we can search for the word "document" immediately followed by either a comma or period by specifying the search string "document[,.]". The special characters in this case are the square brackets, which means that any one of the enclosed characters must follow the word "document".

There are a lot of special characters that can be used in a regular expression. Many of which are listed below:

Character Description
* Match 0 or more characters. Example: h*aven matches both heaven and haven.
+ Match 1 or more characters. Example: he+aven matches heaven and heeaven but not haven.
^ Match what follows only if it's at the beginning of the string. Example: if your regular expression is ^document, it will match document in the string "document the requirements", but it will not match anything in the string "this is a requirements document".
$ Match what precedes only if it's at the end of the string. Example: if your regular expression is document$, it will match document in the string "this is a requirements document", but it will not match anything in the string "document the requirements".
\d Match a digit. Example: \d matches each digit individually in the string "my degree is from 1987". If you want to match all four digits at once, just repeat the \d like \d\d\d\d.
Match any single character. The period is like a wildcard in that it will match anything.
(a|b) Match either the expression represented by 'a' or the expression represented by 'b'. Example: "(1|one) is the loneliest number" will match both strings "1 is the loneliest number" and "one is the loneliest number".

To see regular expressions in action, let's combine some and use the Findstr command to find occurrences of strings in a file. I have a file called Barry.tmp which contains the following text:

Video provides a powerful way to help you prove your point. 
When you click Online Video, you can paste in the embed code for 
the 20 or 30 videos you want to add. You can also type a keyword 
to search online for the video that best fits your document.

Let's use Findstr to find strings of digits in the file: (See Figure 1.)

Figure 1. Searching for strings of digits.

Let me introduce a couple of other concepts. You can use a pair of square brackets to delimit a range of characters to be found. For example, specifying [a-z] will match any alphabetic character. Also noteworthy is that you use the backslash character (\) to "escape" the character that follows and treat it literally. This is useful if you want to find a character that would otherwise be treated as part of the regular expression. Armed with these two pieces of information, we can now search our file for an alphabetic character followed by a period, i.e., find lines that are ends of sentences: (See Figure 2.)

Figure 2. Searching for ends of sentences.

Let me end with a word of caution: In my experience the regular expression engine used at the Windows command line is a bit different from most regular expression engines. Regular expressions that should work (and that do work in other utilities) may not work as expected at the command line. So, knowing your mileage may vary, experiment with them, but don't bet your next paycheck that what you've entered will work as expected.

 This tip (1364) applies to Windows 7, 8, and 10.

Author Bio

Barry Dysert

Barry has been a computer professional for over 35 years, working in different positions such as technical team leader, project manager, and software developer. He is currently a software engineer with an emphasis on developing custom applications under Microsoft Windows. When not working with Windows or writing Tips, Barry is an amateur writer. His first non-fiction book is titled "A Chronological Commentary of Revelation." ...

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Comments

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What is three less than 3?

2021-02-23 12:04:24

Rick Johnson

" . * "dot-star as an adjacent pair should always be described as the RegEx way to match ANY number of ANY characters.
Windows DOS and other command line file search operations have traditionally used a non-RegEx asterisk to match any number of any characters.

This meaning of the asterisk is a core difference between typical and common directory search wildcard(s) and "Regular Expression" notation.


2021-02-23 11:51:00

Rick Johnson

As far as RegEx goes the following description for asterisk was slightly misleading:

"Match 0 or more characters. Example: h*aven matches both heaven and haven"

h*aven does not really match all of "heaven".
h*aven matches every "aven" (including the "aven" in heaven)
and character sequences with any number of 'h's preceding "aven" as in "haven" and "hhhhhaven".

In other words, a RegEx search on "haven and heaven" matches "haven" and "aven".


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