^ (caret): start of line.
$ (dollar): end of line.
[]: character class -- lets you list the characters you want to allow in a point in the match. It can match just a single character in the target text.
- Example:
gr[ea]ymatches "g, followed byr, followed by either aneor ana, all followed byy". - The rules about which characters are and aren’t metacharacters (and exactly what they mean) are different inside a character class.
- Example: within a character class, the character-class metacharacter
-(dash) indicates a range of characters.- Example:
<H[1-6]>is identical to<H[123456]>. - If
-is the first character listed inside the character class (or after[^) the-is interpreted as a literal character.
- Example:
- Example: If
^is the first listed character inside the character class, it "negates" the list of accepted characteres (but, only when it is immediately after the class’s opening bracket).- Example:
[^1-6]matches a character that’s not 1 through 6.
- Example:
- Example: within a character class, the character-class metacharacter
. (dot or point): shorthand for a character class that matches any character.
|: or, matches any one of several subexpressions.
() (parentheses): matches either expression it separates. Used to limit scope of | and to group multiple characters into larger units to which you can apply quantifiers like question mark and star. With tools that support backreferencing, parentheses "remember" the text that the subexpression inside them matches, and the special metasequence \1 represents that text later in the regular expression, whatever it happens to be at the time. Use \1, \2, \3, etc., to refer to the first, second, third, etc. sets.
Backreferencing is a regular-expression feature that allows you to match new text that is the same as some text matched earlier in the expression.
Ignoring differences in capitalization is not a part of the regular-expression language, but is a related useful feature many tools (and programming languages) provide. egrep’s command-line option -i tells it to do a case-insensitive match.
Some versions of egrep offer limited support for word recognition: namely the ability to match the boundary of a word (where a word begins or ends). If some version happens to support them, the metasequences \< and \> could be used. You can think of them as word-based versions of ^ and $ that match the position at the start and end of a word, respectively.
Three reasons for using parentheses are constraining alternation, grouping, and capturing.
? (question mark): optional. It is placed after the character that is allowed to appear at that point in the expression, but whose existence isn’t actually required to still be considered a successful match. It can attach to a parenthesized expression.
- Example:
4(th)?will match4thand4.
+ (plus): one or more of the immediately-preceding item.
* (star): any number, including none, of the immediately-preceding item.
?, + and * are called quantifiers because they influence the quantity of what they govern.
{ min, max }: interval quantifier.
- Example:
{3,12}matches up to 12 times if possible, but settles for three of the immediately-preceding item.
Not many versions of egrep support this notation yet, but many other tools do.
\ (backslash): escape. Used to "escape" all the normal metacharacters. When a metacharacter is escaped, it loses its special meaning and becomes a literal character.
- Example: The metasequence to match an actual period is a period preceded by a backslash:
ega\.att\.comwill matchega.att.com.
The pairing of \ and selected non-metacharacters becomes a metasequence with an implementation-defined meaning (for example, \< often means “start of word”).
The pairing of \ and any other character defaults to simply matching the character (that is, the backslash is ignored).
Knowing the target text well is an important part of wielding regular expressions effectively.