regexp manual page
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Regular Expressions

This page is an excerpt of the UNIX manual page regexp(5) that defines the syntax and semantics of regular expressions used in CADP tools such as exhibitor and OPEN/CAESAR application programming interfaces such as caesar_hide_1 and caesar_rename_1 .

Regular Expressions

A regular expression specifies a set of character strings. A member of this set of strings is said to be matched by the regular expression. Some characters have special meaning when used in a regular expression; other characters stand for themselves.

The following one-character RE s match a single character:

1.1
An ordinary character (not one of those discussed in 1.2 below) is a one-character RE that matches itself.

1.2
A backslash (\) followed by any special character is a one-character RE that matches the special character itself. The special characters are:

a.
., *, [, and \ (period, asterisk, left square bracket, and backslash, respectively), which are always special, except when they appear within square brackets ([]; see 1.4 below).

b.
^ (caret or circumflex), which is special at the beginning of an entire RE (see 4.1 and 4.3 below), or when it immediately follows the left of a pair of square brackets ([]) (see 1.4 below).

c.
$ (dollar sign), which is special at the end of an entire RE (see 4.2 below).

d.
The character used to bound (that is, delimit) an entire RE , which is special for that RE (for example, see how slash (/) is used in the g command, below.)

1.3
A period (.) is a one-character RE that matches any character except new-line.

1.4
A non-empty string of characters enclosed in square brackets ([]) is a one-character RE that matches any one character in that string. If, however, the first character of the string is a circumflex (^), the one-character RE matches any character except new-line and the remaining characters in the string. The ^ has this special meaning only if it occurs first in the string. The minus (-) may be used to indicate a range of consecutive characters; for example, [0-9] is equivalent to [0123456789]. The - loses this special meaning if it occurs first (after an initial ^, if any) or last in the string. The right square bracket (]) does not terminate such a string when it is the first character within it (after an initial ^, if any); for example, []a-f] matches either a right square bracket (]) or one of the ASCII letters a through f inclusive. The four characters listed in 1.2.a above stand for themselves within such a string of characters.

The following rules may be used to construct RE s from one-character RE s:

2.1
A one-character RE is a RE that matches whatever the one-character RE matches.

2.2
A one-character RE followed by an asterisk (*) is a RE that matches 0 or more occurrences of the one-character RE . If there is any choice, the longest leftmost string that permits a match is chosen.

2.3
A one-character RE followed by \{m\}, \{m,\}, or \{m,n\} is a RE that matches a range of occurrences of the one-character RE . The values of m and n must be non-negative integers less than 256; \{m\} matches exactly m occurrences; \{m,\} matches at least m occurrences; \{m,n\} matches any number of occurrences between m and n inclusive. Whenever a choice exists, the RE matches as many occurrences as possible.

2.4
The concatenation of RE s is a RE that matches the concatenation of the strings matched by each component of the RE .

2.5
A RE enclosed between the character sequences \( and \) is a RE that matches whatever the unadorned RE matches.

2.6
The expression \n matches the same string of characters as was matched by an expression enclosed between \( and \) earlier in the same RE . Here n is a digit; the sub-expression specified is that beginning with the n-th occurrence of \( counting from the left. For example, the expression ^\(.*\)\1$ matches a line consisting of two repeated appearances of the same string.

A RE may be constrained to match words.

3.1
\< constrains a RE to match the beginning of a string or to follow a character that is not a digit, underscore, or letter. The first character matching the RE must be a digit, underscore, or letter.

3.2
\> constrains a RE to match the end of a string or to precede a character that is not a digit, underscore, or letter.

An entire RE may be constrained to match only an initial segment or final segment of a line (or both).

4.1
A circumflex (^) at the beginning of an entire RE constrains that RE to match an initial segment of a line.

4.2
A dollar sign ($) at the end of an entire RE constrains that RE to match a final segment of a line.

4.3
The construction ^entire RE $ constrains the entire RE to match the entire line.

The null RE (for example, //) is equivalent to the last RE encountered.

Characters with Special Meaning

Characters that have special meaning except when they appear within square brackets ([]) or are preceded by \ are: ., *, [, \. Other special characters, such as $ have special meaning in more restricted contexts.

The character ^ at the beginning of an expression permits a successful match only immediately after a newline, and the character $ at the end of an expression requires a trailing newline.

Two characters have special meaning only when used within square brackets. The character - denotes a range, [c-c], unless it is just after the open bracket or before the closing bracket, [-c] or [c-] in which case it has no special meaning. When used within brackets, the character ^ has the meaning complement of if it immediately follows the open bracket (example: [^c]); elsewhere between brackets (example: [c^]) it stands for the ordinary character ^.

The special meaning of the \ operator can be escaped only by preceding it with another \, for example \\.


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