re_format(5)                                               re_format(5)

  re_format

  NAME

    re_format - POSIX 1003.2 regular expressions

  DESCRIPTION

    Regular expressions (REs), as defined in POSIX 1003.2, come in two forms:
    basic REs (roughly those of ed(1)) and extended (roughly those of
    egrep(1)). POSIX 1003.2 leaves some aspects of RE syntax and semantics
    open; `|.-' marks decisions on these aspects that might not be fully
    portable to other 1003.2 implementations.

  Basic regular expressions

    A basic regular expression is one or more pieces concatenated. It matches
    a match for the first, followed by a match for the second, and so on.

    A piece is an atom possibly followed by a single|.- `*' or bound. An atom
    followed by `*' matches a sequence of 0 or more matches of the atom.

    A bound is `\{' followed by an unsigned decimal integer, possibly followed
    by `,' possibly followed by another unsigned decimal integer, always
    followed by `\}'. The integers must lie between 0 and RE_DUP_MAX (255|.-
    ) inclusive, and if there are two of them, the first cannot exceed the
    second. An atom followed by a bound containing one integer i and no comma
    matches a sequence of exactly i matches of the atom. An atom followed by a
    bound containing one integer i and a comma matches a sequence of i or more
    matches of the atom. An atom followed by a bound containing two integers i
    and j matches a sequence of i through j (inclusive) matches of the atom.

    An atom is a regular expression enclosed in `\(\)' (matching a match for
    the regular expression), a bracket expression (see later in this topic),
    `.' (matching any single character), `^' (matching the null string at the
    beginning of a line), `$' (matching the null string at the end of a line),
    a `\' followed by one of the characters `^.[$*\' (matching that character
    taken as an ordinary character), a `\' followed by any other character|.-
    (matching that character taken as an ordinary character, as if the `\' had
    not been present|.-), a back reference, or a single character with no
    other significance (matching that character). It is illegal to end an RE
    with `\'.

    The character `^' is an ordinary character, except at the beginning of the
    RE or|.- the beginning of a parenthesized subexpression; the character `$'
    is an ordinary character except at the end of the RE or|.- the end of a
    parenthesized subexpression; the character `*' is an ordinary character if
    it appears at the beginning of the RE or the beginning of a parenthesized
    subexpression (after a possible leading `^').

    A back reference is `\' followed by a non-zero decimal digit d. It matches
    the same sequence of characters matched by the
    dthparenthesizedsubexpression (numbering subexpressions by the positions
    of their opening parentheses, left to right), such as, for instance
    `\([bc]\)\1' matches `bb' or `cc' but not `bc'.

  Extended regular expressions

    As the name suggests, extended regular expressions extend the capabilities
    of basic regular expressions by adding several new constructs and by
    eliminating back references.

    The following new characters are defined for extended regular expressions:

    + ? { } ( ) |

    An atom is now is a regular expression enclosed in `()' instead of `\(\)',
    an empty set of `()' (matching the null string)|.-, a bracket expression
    (see later in this topic), `.' (matching any single character), `^'
    (matching the null string at the beginning of a line), `$' (matching the
    null string at the end of a line), a `\' followed by one of the characters
    `^.[$()|*+?{\' (matching that character taken as an ordinary character), a
    `\' followed by any other character|.- (matching that character taken as
    an ordinary character, as if the `\' had not been present|.-), or a single
    character with no other significance (matching that character). A `{'
    followed by a character other than a digit is an ordinary character, not
    the beginning of a bound|.-. You cannot end an RE with `\'.

    An atom followed by `*' matches a sequence of 0 or more matches of the
    atom. An atom followed by `+' matches a sequence of 1 or more matches of
    the atom. An atom followed by `?' matches a sequence of 0 or 1 matches of
    the atom.

    The delimiter characters for a bound are `{' and `}' instead of `\{' and
    `\}'.

    The `|' character separates branches An extended regular expression
    matches anything that matches one of the branches. For example,
    `alpha|beta' matches either `alpha' or `beta'.

    Basic pattern     Extended pattern     Matches

    c                 c                    Matches the character c except as
                                           below

    \c                \c                   The character c

    ^                 ^                    The beginning of a line

    $                 $                    The end of a line

    .                 .                    Any single character

    [list]            [list]               Any one of the characters in list

    [^list]           [^list]              Any one of the characters not in
                                           list

    ()                ()                   The null string

    \(a\)             (a)                  Groups pattern a into a single
                                           expression

    a*                a*                   Zero or more occurrences of a

                      a+                   One or more occurrences of a

                      a?                   Zero or one occurrence of a

    a\{n\}            a{n}                 N occurrences of a

    a\{n,\}           a{n,}                N or more occurrences of a

    a\{n,m\}          a{n,m}               Between n and m occurrences of a

                      a|b                  Pattern a or pattern b

    \d                                     Refers back to matched expression
                                           d

  Bracket expressions

    A bracket expression is a list of characters enclosed in brackets: []. It
    normally matches any single character from the list (but see later in this
    topic). If the list begins with `^', it matches any single character (but
    see later in this topic) not from the rest of the list. If two characters
    in the list are separated by `-', this is shorthand for the full range of
    characters between those two (inclusive) in the collating sequence; for
    example, `[0-9]' in ASCII matches any decimal digit. It is illegal|.- for
    two ranges to share an endpoint; for example, `a-c-e'. Ranges are very
    collating-sequence-dependent, and portable programs should avoid relying
    on them.

    To include a literal `]' in the list, make it the first character
    (following a possible `^'). To include a literal `-', make it the first or
    last character, or the second endpoint of a range. To use a literal `-' as
    the first endpoint of a range, enclose it in `[.' and `.]' to make it a
    collating element (see later in this topic). With the exception of these
    and some combinations using `[' (see next paragraphs), all other special
    characters, including `\', lose their special significance within a
    bracket expression.

    Within a bracket expression, a collating element (a character, a
    multicharacter sequence that collates as if it were a single character, or
    a collating-sequence name for either) enclosed in `[.' and `.]' stands for
    the sequence of characters of that collating element. The sequence is a
    single element of the bracket expression's list. A bracket expression
    containing a multicharacter collating element can thus match more than one
    character. For instance, if the collating sequence includes a `ch'
    collating element, the RE `[[.ch.]]*c' matches the first five characters
    of `chchcc'.

    Within a bracket expression, a collating element enclosed in `[=' and `=]'
    is an equivalence class, standing for the sequences of characters of all
    collating elements equivalent to that one, including itself. (If there are
    no other equivalent collating elements, the treatment is as if the
    enclosing delimiters were `[.' and `.]'.) For example, if o and o.^ are
    the members of an equivalence class, `[[=o=]]', `[[=o.^=]]', and `[oo.^]'
    are all synonymous. An equivalence class cannot |.- be an endpoint of a
    range.

    Within a bracket expression, the name of a character class enclosed in `[:
    ' and `:]' stands for the list of all characters belonging to that class.
    Standard character class names are:

    alnum     digit     punct

    alpha     graph     space

    blank     lower     upper

    cntrl     print     xdigit

    These stand for the character classes defined in ctype(). A locale can
    provide others. A character class cannot be used as an endpoint of a
    range.

    There are two special cases|.- of bracket expressions: the bracket
    expressions `[[:<:]]' and `[[:>:]]' match the null string at the beginning
    and end of a word respectively. A word is defined as a sequence of word
    characters that is neither preceded nor followed by word characters. A
    word character is an alnum character (as defined by ctype()) or an
    underscore. This is an extension, compatible with but not specified by
    POSIX 1003.2, and should be used with caution in software intended to be
    portable to other systems.

    In the event that an RE could match more than one substring of a given
    string, the RE matches the one starting earliest in the string. If the RE
    could match more than one substring starting at that point, it matches the
    longest. Subexpressions also match the longest possible substrings,
    subject to the constraint that the whole match be as long as possible,
    with subexpressions starting earlier in the RE taking priority over ones
    starting later. Note that higher-level subexpressions thus take priority
    over their lower-level component subexpressions.

    Match lengths are measured in characters, not collating elements. A null
    string is considered longer than no match at all. For example, `bb*'
    matches the three middle characters of `abbbc',
    `(wee|week)(knights|nights)' matches all ten characters of `weeknights';
    when `(.*).*' is matched against `abc', the parenthesized subexpression
    matches all three characters; and when `(a*)*' is matched against `bc'
    both the whole RE and the parenthesized subexpression match the null
    string.

    If case-independent matching is specified, the effect is much as if all
    case distinctions had vanished from the alphabet. When an alphabetic that
    exists in multiple cases appears as an ordinary character outside a
    bracket expression, it is effectively transformed into a bracket
    expression containing both cases; for example, `x' becomes `[xX]'. When it
    appears inside a bracket expression, all case counterparts of it are added
    to the bracket expression, so that, for example, `[x]' becomes `[xX]' and
    `[^x]' becomes `[^xX]'.

    No particular limit is imposed on the length of REs|.-. Programs intended
    to be portable should not employ REs longer than 256 bytes because an
    implementation can refuse to accept such REs and remain POSIX-compliant.

  BUGS

    The current 1003.2 spec says that `)' is an ordinary character in the
    absence of an unmatched `('; this was an unintentional result of a wording
    error, and change is likely. Avoid relying on it.

    1003.2's specification of case-independent matching is vague. The "one
    case implies all cases" definition given above is current consensus among
    implementers as to the right interpretation.

    The syntax for word boundaries is incredibly ugly.

  SEE ALSO

    ctype()

    regex(3)

    POSIX 1003.2, section 2.8 (Regular Expression Notation).