Vertical bar

Punctuation marks
apostrophe (' ) parentheses ( ( ) ), brackets ( [ ] ); ( { } ); ( < > ) colon ( : ) comma ( , ) dash ( ‒ ); ( – ); ( — ); ( ― ) ellipsis ( … ) ( ... ) exclamation mark ( ! ); ( ¡ ! ) full stop/period ( . ) hyphen ( - ); ( ‐ ) interrobang ( ‽ ) question mark ( ? ); ( ¿ ? ) quotation marks ( ‘ ’ ); ( “ ” );     ( ‚ ’ ); ( „ ” ); ( ‚ ‘ ); ( „ “ );     ( ‹ › ); ( « » ); ( › ‹ ); ( » « );     ( 「 」 ); ( 『 』 ) semicolon ( ; ) slash ( / ) and backslash ( \ ) space (   ) and interpunct ( · )
ampersand ( & ) asterisk ( * ) and dagger ( † ‡) bullet ( •, more ) commercial at ( @ ) number sign ( # ) prime ( ′ ) and double prime (″) tilde ( ~ ) underscore ( _ ) vertical bar / pipe ( | )

Vertical bar or pipe is the name of the ASCII character at position 124 (decimal). The character is depicted as either a solid vertical bar ("|") or a vertical bar with a break in the middle (broken bar "¦"). The character is usually depicted as a broken bar on IBM PC keyboards to distinguish it from other characters. However, today the "broken bar" is a separate character, U+00A6 (¦).

Usage:

Mathematics

The vertical bar is used as a mathematical symbol in

• absolute value: <math>|x|<math>, read "the absolute value of x".
• norms: <math>||(x_1,x_2)||<math>, read "the norm of x sub one, x sub two".
• set definition <math>\{x|x<2\}<math>, read "the set of x such that x is less than two". Often sets are written with a colon ':' rather than a vertical bar.
• conditional probability: <math>P(X|Y)<math>, read "the probability of X given Y".

Cursors

In text that cannot be edited but can be selected, for example, on a web page, the cursor can also be a vertical bar, possibly with little cross bars at the top and bottom.

Wiki markup

The vertical bar is used in internal links to separate a link from the displayed text. E.g., [[c programming language|C]] displays as C.

Backus-Naur form

In BNF the expression consists of sequences of symbols and/or sequences separated by '|', indicating a choice, the whole being a possible substitution for the symbol on the left.

<personal-name> ::= <name> | <initial>

Unix

A pipe is an operating system mechanism originating in Unix which allows the output of one process to be used as input to another. Used in a shell command, this is represented by the vertical bar character. See Pipe (Unix).

Regular Expression

In regular expressions, the vertical bar indicates alternation. E.g., grep 'foo|bar' matches lines containing 'foo' or 'bar'.

Disjunction

In the C, perl, and many others, it is used to designate bitwise or a|b or doubled up logical or a||b.

Etymology

The name pipe for the character "|" came into common usage due to the fact that it is used in unix to pipe the output of one program into another program.