Regex Quick Reference

Luke Hebert

January 24, 2022 - 2 minutes read - 305 words

Regex patterns can feel like a foreign language until you build a small, reliable set of go-to snippets. The patterns below cover the most common tasks I reach for in shell scripts and log analysis: extracting email addresses, using lookarounds, and matching specific character classes. Keep this page handy as a quick reference while you work.

Find Email Addresses

A reliable one-liner for pulling email addresses out of a log file:

egrep -o '[A-Za-z0-9.-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}' email.log | sort | uniq

The -o flag prints only the matched portion, and piping through sort | uniq removes duplicates.

Forward and Backward Lookups

Perl-compatible lookarounds let you match text based on what surrounds it without consuming that surrounding text:

grep -oP '(?<=<)\S*(?=>:)'

This example extracts the content between < and >: without including the delimiters themselves.

Special Meta Characters

Pattern	Meaning
`^`	Line starts with
`$`	Line ends with

Meta Characters

Pattern	Meaning
`[]`	Character class (single characters)
`.`	Matches any character except newline
`()`	Grouping (full expressions)
`\|`	Or (alternation)

Qualifiers

Pattern	Meaning
`?`	Optional match (zero or one)
`+`	One or more of the preceding element
`*`	Any number including zero of the preceding element
`{2,5}`	Specify length (2 to 5 characters)

Perl Look Ahead and Behind (grep -P)

These require the -P flag to enable Perl-compatible regex:

Pattern	Meaning
`(?=blah)`	Positive look ahead
`(?<=blah)`	Positive look behind
`(?:blah)`	Non-capture group
`.*?`	Lazy match instead of greedy

Character Shorthand Classes

Pattern	Meaning	Equivalent
`\d`	A digit	`[0-9]`
`\D`	A non-digit	`[^0-9]`
`\w`	A word character	`[a-zA-Z0-9_]`
`\W`	A non-word character	`[^a-zA-Z0-9_]`
`\s`	A whitespace character	`[ \t\n\r\f]`
`\S`	A non-whitespace character	`[^ \t\n\r\f]`

Tip

When in doubt, test your pattern against a small sample file first. The grep -oP combination is the fastest way to iterate on a pattern before committing it to a script.