Introducing Regular Expressions

Regular Expression Glossary

anchor

Specifies a location in a line or string. For example, the caret or circumflex character (^) signifies the beginning of a line or string of characters, and the dollar sign character ($), the end of a line or string.

alternation

Separating a list of regular expressions with a vertical bar (|) character, indicating or. In other words, match any of the regular expressions separated by one or more | characters. In some applications, such as grep or sed that use basic regular expressions (BREs), the | is preceded by a backslash, as in \|. See also basic regular expressions.

ASCII

American Standard Code for Information Interchange. A 128-character encoding scheme for English (Latin) characters developed in the 1960s. See also Unicode.

assertions

See zero-width assertions.

atom

See metacharacter.

atomic group

A grouping that turns off backtracking when a regular expression inside (?>…) fails to match. See also backtracking, groups.

backreference

Refers to a previous regular expression captured with parentheses using a reference in the form of \1, \2, and so forth.

backtracking

Stepping back, character by character, through an attempted match to find a successful match. Used with a greedy match, but not a lazy or possessive match. Catastrophic backtracking occurs when a regex processor makes perhaps thousands of attempts to make a match and consumes a vast amount (read most) of the computing resources available. One way to avoid catastrophic backtracking is with atomic ...