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8.4. Extended Regular Expressions
The metacharacters available in vi's
search and substitution regular expressions are described in
Section 6.3.1 in Chapter 6.
Each of the clones provides some form of extended regular
expressions, either as an option or always available.
Typically these are the same (or almost the same) as what's provided
by egrep. Unfortunately, each one's
extended flavor is slightly different from the others'.
To give you a feel for what extended regular expressions can
do, we present them
in the context of nvi.
Each clone's chapter then describes that editor's
extended syntax, without repeating the examples.
nvi extended regular expressions are the Extended Regular
Expressions (EREs) as defined by the POSIX standard.
In order to enable this feature, use set extended
from either your .nexrc file
or from the ex colon prompt.
Besides the standard metacharacters described in
Chapter 6, and the POSIX bracket expressions mentioned in
Section 6.3.2 in the same chapter,
the following metacharacters are available:
- |
-
Indicates alternation.
For example, a|b matches either
a or b.
However, this construct is not limited to single characters:
house|home matches either
of the strings house or home.
- (...)
-
Used for grouping, to allow the application
of additional regular expression operators. For example, house|home
can be shortened (if not simplified) to ho(use|me).
The *
operator can be applied to text in parentheses:
(house|home)*
matches
home,
homehouse,
househomehousehouse
and so on.
When extended is set, text grouped
with parentheses acts like text grouped in \(...\)
in regular vi; the actual text matched can be
retrieved in the replacement part of a substitute command with
\1, \2, etc.
In this case, \( represents a literal left parenthesis.
- +
-
Matches one or more of the preceding regular expressions.
This is either a single character, or a group of characters enclosed
in parentheses.
Note the difference between + and *.
The * is allowed to match nothing, but with
+ there must be at least one match.
For example, ho(use|me)* matches ho
as well as home and house, but
ho(use|me)+ will not match ho.
- ?
-
Matches zero or one occurrence of the preceding regular expression.
This indicates "optional" text that is either present or
not present. For example, free?d will match
either fred or freed, but
nothing else.
- {...}
-
Defines an interval expression.
Interval expressions describe counted numbers of repetitions.
In the description below, n and m
represent integer constants.
- {n}
- Matches exactly n repetitions of the
previous regular expression. For example, (home|house){2}
matches
homehome,
homehouse,
househome,
and
househouse,
but nothing else.
- {n,}
- Matches n or more repetitions of the
previous regular expression.
Think of it as "as least n"
repititions.
- {n,m}
- Matches n to m repititions.
The bounding is important, since it controls how much text would be
replaced during a substitute command.[42]
When extended is not set, nvi
provides the same functionality with \{
and \}.
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