Regular Expressions

A regular expression (or regex for short) is a standard way of using text to form a search to match patterns.

Similar to using an asterisk like this: *.jpg in a search box to find all JPEG files, you can use a regular expression (along with something like grep) to match much more complex patterns.

For example, you could use:

\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}\b

to search for any e-mail addresses in a file

Cheat Sheet

a - Literal character, like the letter, "a". Every character is literal except these twelve: \ ^ $ . | ? * + ( ) [ {

. (dot) - a single character.

? - the preceding character matches 0 or 1 times only.

* - the preceding character matches 0 or more times.

+ - the preceding character matches 1 or more times.

{n} - the preceding character matches exactly n times.

{n,m} - the preceding character matches at least n times and not more than m times. Example: a{2,4} match the character at least twice, but not more than four times.

[agd] - the character is one of those included within the square brackets.

[^agd] - the character is not one of those included within the square brackets.

[c-f] - the dash within the square brackets operates as a range. In this case it means either the letters c, d, e or f. You can use numbers to specify a range of numbers as well.

() - allows us to group several characters to behave as one.

| (pipe symbol) - the logical OR operation.

^ - matches the beginning of the line.

$ - matches the end of the line.

\ - escapes a special character. For example, if you want to see if a file has a question mark in it, you can't use the question mark symbol because it has a special meaning. So, we escape (tell regex to ignore it's special meaning and treat it as a literal character) it by putting a backslash in front of it. Like this: \?

Anchor Characters

Regular expressions examine the text between separators. If you want to search for a pattern that is at one end or the other, you use anchors. The character ^ is the starting anchor, and the character $ is the end anchor.
Note that ^ and $ are only anchors if the are used at the start (^) or end ($) of a pattern.

Examples: Pattern Matches
^A "A" at the beginning of a line
A$ "A" at the end of a line
A^ "A^" anywhere on a line
$A "$A" anywhere on a line
^^ "^" at the beginning of a line
$$ "$" at the end of a line

References

1. Ryan's Tutorials Grep and Regular Expressions
2. Regular Expressions Info
3. grymoire Regular Expressions tutorial