regexp(5)

A bracket expression is either a matching list expression or a non-matching list expression, and consists of one or more expressions in any order. Expressions can be: collating elements, collating symbols, noncollating characters, equivalence classes, range expressions, or character classes. The right bracket (]) loses its special meaning and represents itself in a bracket expression if it occurs first in the list (after an initial ^, if any). Otherwise, it terminates the bracket expression (unless it is the ending right bracket for a valid collating symbol, equivalence class, or character class, or it is the collating element within a collating symbol or equivalence class expression). The special characters

(period, asterisk, left bracket, and backslash) lose their special meaning within a bracket expression.

The character sequences:

(left-bracket followed by a period, equal-sign or colon) are special inside a bracket expression and are used to delimit collating symbols, equivalence class expressions and character class expressions. These symbols must be followed by a valid expression and the matching terminating .], =], or :].

A matching list expression specifies a list that matches any one of the characters represented in the list. The first character in the list cannot be the circumflex. For example, [abc] is an RE that matches any of a, b, or c.

A non-matching list expression begins with a circumflex (^), and specifies a list that matches any character or collating element except newline and the characters represented in the list. For example, [^abc] is an RE that matches any character except newline or a, b, or c. The circumflex has this special meaning only when it occurs first in the list, immediately following the left square bracket.

A collating element is a sequence of one or more characters that represents a single element in the collating sequence as identified via the most current setting of the locale variable LC_COLLATE (see setlocale(3C)).

A collating symbol is a collating element enclosed within bracket-period ([. .]) delimiters. Multicharacter collating elements must be represented as collating symbols to distinguish them from single-character collating elements. For example, if the string ch is a valid collating element, then [[.ch.]] is treated as an element matching the same string of characters, while ch is treated as a simple list of the characters c and h. If the string within the bracket-period delimiters is not a valid collating element in the current collating sequence definition, the symbol is treated as an invalid expression.

A noncollating character is a character that is ignored for collating purposes. By definition, such characters cannot participate in equivalence classes or range expressions.

An equivalence class expression represents the set of collating elements belonging to an equivalence class. It is expressed by enclosing any one of the collating elements in the equivalence class within bracket-equal ([= =]) delimiters. For example, if a and A belong to the same equivalence class, then [[=a=]b] and [[=A=]b] are each equivalent to [aAb].

A range expression represents the set of collating elements that fall between two elements in the current collation sequence as defined via the most current setting of the locale variable LC_COLLATE (see setlocale(3C)). It is expressed as the starting point and the ending point separated by a hyphen (-).

The starting range point and the ending range point must be a collating element, collating symbol, or equivalence class expression. An equivalence class expression used as an end point of a range expression is interpreted such that all collating elements within the equivalence class are included in the range. For example, if the collating order is A, a, B, b, C, c, ch, D, and d and the characters A and a belong to the same equivalence class, then the expression [[=a=]-D] is treated as [AaBbCc[.ch.]D].

Both starting and ending range points must be valid collating elements, collating symbols, or equivalence class expressions, and the ending range point must collate equal to or higher than the starting range point; otherwise the expression is invalid. For example, with the above collating order and assuming that E is a noncollating character, then both the expressions [[=A=]-E] and [d-a] are invalid.

An ending range point can also be the starting range point in a subsequent range expression. Each such range expression is evaluated separately. For example, the bracket expression [a-m-o] is treated as [a-mm-o].

The hyphen character is treated as itself if it occurs first (after an initial ^, if any) or last in the list, or as the rightmost symbol in a range expression. As examples, the expressions [-ac] and [ac-] are equivalent and match any of the characters a, c, or -; the expressions [^-ac] and [^ac-] are equivalent and match any characters except newline, a, c, or -; the expression [%--] matches any of the characters in the defined collating sequence between % and - inclusive; the expression [--@] matches any of the characters in the defined collating sequence between - and @ inclusive; and the expression [a--@] is invalid, assuming - precedes a in the collating sequence.

If a bracket expression must specify both - and ], the ] must be placed first (after the ^, if any) and the - last within the bracket expression.

A character class expression represents the set of characters belonging to a character class, as defined via the most current setting of the locale variable LC_CTYPE. It is expressed as a character class name enclosed within bracket-colon ([: :]) delimiters.

Standard character class expressions supported in all locales are:

For example, if the locale variable LC_CTYPE is set to C, the expression [[:upper:]] is equivalent to [A-Z]. Similarly the expression [[:digit:]] is same as [0-9].

The concatenation of REs is an RE that matches the first encountered concatenation of the strings matched by each component of the RE. For example, the RE bc matches the second and third characters of the string abcdefabcdef.

An RE matching a single character followed by an asterisk (*) is an RE that matches zero or more occurrences of the RE preceding the asterisk. The first encountered string that permits a match is chosen, and the matched string will encompass the maximum number of characters permitted by the RE. For example, in the string abbbcdeabbbbbbcde, both the RE b*c and the RE bbb*c are matched by the substring bbbc in the second through fifth positions. An asterisk as the first character of an RE loses this special meaning and is treated as itself.

A subexpression can be defined within an RE by enclosing it between the character pairs \( and \). Such a subexpression matches whatever it would have matched without the \( and \). Subexpressions can be arbitrarily nested. An asterisk immediately following the \( loses its special meaning and is treated as itself. An asterisk immediately following the \) is treated as an invalid character.

The expression \n matches the same string of characters as was matched by a subexpression enclosed between \( and \) preceding the \n. The character n must be a digit from 1 through 9, specifying the n-th subexpression (the one that begins with the n-th \( and ends with the corresponding paired \). For example, the expression ^\(.*\)\1$ matches a line consisting of two adjacent appearances of the same string.

If the \n is followed by an asterisk, it matches zero or more occurrences of the subexpression referred to. For example, the expression \(ab\(cd\)ef\)Z\2*Z\1 matches the string abcdefZcdcdZabcdef.

An RE matching a single character followed by \{m\}, \{m,\}, or \{m,n\} is an RE that matches repeated occurrences of the RE. The values of m and n must be decimal integers in the range 0 through 255, with m specifying the exact or minimum number of occurrences and n specifying the maximum number of occurrences. \{m\} matches exactly m occurrences of the preceding RE, \{m,\} matches at least m occurrences, and \{m,n\} matches any number of occurrences between m and n, inclusive.

The first encountered string that matches the expression is chosen; it will contain as many occurrences of the RE as possible. For example, in the string abbbbbbbc the RE b\{3\} is matched by characters two through four, the RE b\{3,\} is matched by characters two through eight, and the RE b\{3,5\}c is matched by characters four through nine.

A concatenation of EREs matches the first encountered concatenation of the strings matched by each component of the ERE. Such a concatenation of EREs enclosed in parentheses matches whatever the concatenation without the parentheses matches. For example, both the ERE bc and the ERE (bc) matches the second and third characters of the string abcdefabcdef. The longest overall string is matched.

The special character plus (+), when following an ERE matching a single character, or a concatenation of EREs enclosed in parenthesis, is an ERE that matches one or more occurrences of the ERE preceding the plus sign. The string matched will contain as many occurrences as possible. For example, the ERE b+c matches the fourth through seventh characters in the string acabbbcde.

The special character asterisk (*), when following an ERE matching a single character, or a concatenation of EREs enclosed in parenthesis, is an ERE that matches zero or more occurrences of the ERE preceding the asterisk. For example, the ERE b*c matches the first character in the string cabbbcde. If there is any choice, the longest left-most string that permits a match is chosen. For example, the ERE b*cd matches the third through seventh characters in the string cabbbcdebbbbbbcdbc.

The special character question mark (?), when following an ERE matching a single character, or a concatenation of EREs enclosed in parenthesis, is an ERE that matches zero or one occurrences of the ERE preceding the question mark. The string matched will contain as many occurrences as possible. For example, the ERE b?c matches the second character in the string acabbbcde.

interval expression that functions the same way as basic regular expression syntax, ERE\{m,n\}

square brackets

asterisk, plus sign, question mark

anchoring

concatenation

alternation

The asterisk (*) is a pattern that matches any string, including the null string.

The concatenation of patterns matching a single character is a valid pattern that matches the concatenation of the single characters or collating elements matched by each of the concatenated patterns. For example, the pattern a[bc] matches the string ab and ac.

The concatenation of one or more patterns matching a single character with one or more asterisks is a valid pattern. In such patterns, each asterisk matches a string of zero or more characters, up to the first character that matches the character following the asterisk in the pattern.

For example, the pattern a*d matches the strings ad, abd, and abcd; but not the string abc. When an asterisk is the first or last character in a pattern, it matches zero or more characters that precede or follow the characters matched by the remainder of the pattern. For example, the pattern a*d* matches the strings ad, abcd, abcdef, aaaad, and adddd; the pattern *a*d matches the strings ad, abcd, efabcd, aaaad, and adddd.