Regular Expressons

Regular expressions, or “regex” are a very powerful tool for searching and processing text. At a high level, regular expressions are a way of defining patterns of text to operate on. Certain tools use regular expressions to take these patterns and do something with them.

Regex Syntax

Unfortunately, the syntax of regular expressions is not standardized. Every tool that includes support for regular expressions implements a different set of features and syntax. Luckily, there are some very common themes that appear in most implementations. Where there is ambiguity between implementations, though, we’ll use the syntax used by grep and sed, two Unix tools that make heavy use of regular expressions

TL;DR

If you take only two things away from this discussion of regexs, let them be this:

• .: matches any single character (i.e., once)
• <pattern>*: matches <pattern> zero or more times

Read on to learn what these things mean and why they’re cool!

Also, a lot of special regex characters are also special bash characters. To ensure that your regex gets passed along to grep and sed in tact, you’ll almost always want to surround it in single quotes. See Strings for more information.

Plain Patterns

There are a ton of special characters that you can use in regular expressions, which we’ll take a look at later. If you don’t use any of these characters (like plain word characters, numbers, some punctuation), the regular expression will match exactly what you typed.

Note: Most regular expressions are case sensitive by default.

Quantifiers (*, \{n,m\})

Now we’re getting into the special characters. Some special characters allow you to specify how many times a pattern should be repeated when trying to perform a match.

• <pattern>*: matches zero or more occurrences of <pattern>
• <pattern>\{n,m\}: matches at least n and at most m occurrences of <pattern>
  • if n is omitted, it is assumed to be 0
  • if m is omitted, it is assumed to be infinity

By default, regexs are greedy: if the number of times they can repeat and still match a pattern is ambiguous, they consume as many characters as they can.

Note: Single characters are treated as whole patterns. If the pattern you want to repeat has more than one character, wrap the pattern in \(...\).

Character Classes ([...])

Sometimes, we want a pattern to match any character from a set of characters. To do this, we can define character classes. Simply put all the characters that you want to potentially match inside [...]. You can also use hyphens (-) to specify all characters within a range of characters.

Note: Most characters which would otherwise be special characters do not need to be escaped inside a character class.

There are a lot of handy, “prebuilt” character classes that you can use without using square braces:

For more character classes, see man 7 re_format.

And more!

It would take far longer than this to fully list every regex feature, but those listed here are often more than enough. For more information on regexs, Googling helps a lot. And please, don’t try to memorize regex syntax, except for . and *! It’s best just to look up the syntax for whatever tool you’re using when you’re using it.

Searching the Text of Files (grep)

The first tool we’ll talk about, grep, is used to search the contents of files for lines matching a regular expression. It’s syntax is

$ grep <regex> [<file> ...]

Including one or more files is optional; if left out, grep accepts it’s input on stdin.

Normally, because the special characters used in regular expressions are often special characters in bash too, it’s best to enclose the pattern in single quotes.

Replacing Text (sed)

Another common thing to do is to replace text matching a certain pattern with another string. The Unix tool sed has a feature that lets us accomplish this. (In fact, sed can do many more things, but we’ll just be looking at the find and replace features). It’s syntax (for our use cases specifically) is:

# Replace the first occurence of "<find>" on each line with "<replace>"

$ sed -e 's/<find>/<replace>/' [<file> ...]

# Replace the all occurences of "<find>" on each line with "<replace>"

$ sed -e 's/<find>/<replace>/g' [<file> ...]

As with grep, including one or more files is optional. It reads from stdin when no file is specified.

Example

There are tons of use cases for sed and grep, so to give you a taste of how it works, we’ll walk through a single, real world example: parsing the names of the course staff from a text file.

Let’s say I have the data about the instructors in the following format, but all I want are their names.

# file: staff.yml

staff:

  - id: jzimmerm

    domain: andrew

    name: Josh Zimmerman

  - id: jxc

    domain: cs

    name: Jacobo Carrasquel

  - id: nmunson

    domain: andrew

    name: Nick Munson

  - id: jezimmer

    domain: andrew

    name: Jake Zimmerman

  - id: dringwal

    domain: andrew

    name: Dan Ringwalt

  - id: mjmurphy

    domain: andrew

    name: Michael Murphy

Interestingly, all the relevant lines start with the same pattern! First let’s see if we can just print these lines. By searching for all lines that match the pattern name: with grep, we can filter out lines that we don’t even want to consider:

$ grep 'name:' staff.yml
    name: Josh Zimmerman
    name: Jacobo Carrasquel
    name: Nick Munson
    name: Jake Zimmerman
    name: Dan Ringwalt
    name: Michael Murphy

Cool, now we just need to get rid of everything but the actual names. If you notice, there are a few parts that each line has in common: some arbitrary amount of whitespace at the beginning of the line, the literal text ‘name: ‘, and the actual name. We want to get rid of the first two of these parts, which we can do using sed:

# pipe output of grep into sed

$ grep 'name:' staff.yml | sed -e 's/^ *name: //'
Josh Zimmerman
Jacobo Carrasquel
Nick Munson
Jake Zimmerman
Dan Ringwalt
Michael Murphy

Whoa, that’s it! Let’s break down what we just did.

First, we piped the output of our previous grep command into sed. This is so that we only perform substitution on the lines that it makes sense to—the lines containing ‘name:’.

Then, we used sed to find a replace a pattern. Notice that the replacement pattern is empty (there’s nothing between the //). We do this because deleting text is the same as finding and replacing with nothing.

Now let’s look at the pattern we crafted for sed: /^ *name: /. The first character is something we haven’t seen yet. It just matches the beginning of a line, so we know that the text we find doesn’t come in the middle of a word or name. Next, we say to match a space, repeated zero or more times. This takes care of matching all the indentation in the file. After this we match 'name: ' just like with grep.

Finally, by replacing this pattern with nothing, we’ve removed everything but the names, which is what we wanted.

References

This page has been far from comprehensive. Here are some resources that can help when learning how to use regexs:

• Regular Expressions 101
  • Online regular expression visualizer and quick reference
  • Explains why a regex works the way it does
  • Quick reference of common patterns and examples
  • support for PRCE (Perl-compatible regex), Python, and JavaScript flavors
  • Neither grep nor sed flavors, but close
• Regular-Expressions.info
  • Start with the QuickStart
  • In depth tutorial about regular expressions
• man 7 re_format
  • Man page for regular expression formats

Going Further

• Sed - An Introduction and Tutorial
  • A very comprehensive but readable introduction to sed