path: root/doc/regexp.n

'\"
'\" Copyright (c) 1998 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
'\" SCCS: @(#) regexp.n 1.15 98/01/22 16:51:23
'\" 
.so man.macros
.TH regexp n 8.1 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
regexp \- Match a regular expression against a string

.SH SYNOPSIS
\fBregexp \fR?\fIswitches\fR? \fIexp string \fR?\fImatchVar\fR? ?\fIsubMatchVar subMatchVar ...\fR?
.BE

.SH DESCRIPTION
.PP
Determines whether the regular expression \fIexp\fR matches part or
all of \fIstring\fR and returns 1 if it does, 0 if it doesn't.
.LP
If additional arguments are specified after \fIstring\fR then they
are treated as the names of variables in which to return
information about which part(s) of \fIstring\fR matched \fIexp\fR.
\fIMatchVar\fR will be set to the range of \fIstring\fR that
matched all of \fIexp\fR.  The first \fIsubMatchVar\fR will contain
the characters in \fIstring\fR that matched the leftmost parenthesized
subexpression within \fIexp\fR, the next \fIsubMatchVar\fR will
contain the characters that matched the next parenthesized
subexpression to the right in \fIexp\fR, and so on.
.LP
If the initial arguments to \fBregexp\fR start with \fB\-\fR then
they are treated as switches.  The following switches are
currently supported:
.TP 10
\fB\-nocase\fR
Causes upper-case characters in \fIstring\fR to be treated as
lower case during the matching process.
.TP 10
\fB\-indices\fR
Changes what is stored in the \fIsubMatchVar\fRs. 
Instead of storing the matching characters from \fBstring\fR,
each variable
will contain a list of two decimal strings giving the indices
in \fIstring\fR of the first and last characters in the matching
range of characters.
.TP 10
\fB\-\|\-\fR
Marks the end of switches.  The argument following this one will
be treated as \fIexp\fR even if it starts with a \fB\-\fR.
.LP
If there are more \fIsubMatchVar\fR's than parenthesized
subexpressions within \fIexp\fR, or if a particular subexpression
in \fIexp\fR doesn't match the string (e.g. because it was in a
portion of the expression that wasn't matched), then the corresponding
\fIsubMatchVar\fR will be set to ``\fB\-1 \-1\fR'' if \fB\-indices\fR
has been specified or to an empty string otherwise.

.SH DIFFERENT FLAVORS OF REs
.VS 8.1
Regular expressions (``RE''s),
as defined by POSIX, come in two flavors:
\fIextended\fR
REs (roughly those of
\fIegrep\fR),
``EREs'',
and
\fIbasic\fR
REs (roughly those of
\fIed\fR),
``BREs''.
This implementation adds a third flavor,
\fIadvanced\fR
REs, ``AREs'', that offer a syntax for specifying BREs and EREs.  The
following primarily describes AREs.
.PP
BREs mostly exist for backward compatibility in some old programs;
they will be discussed at the end.
POSIX EREs are
\fIalmost\fR
an exact subset of AREs;

.SH "REGULAR EXPRESSION SYNTAX"
.PP
Regular expressions are implemented using the package written by Henry
Spencer, based on the 1003.2 spec and some (not quite all) of the
Perl5 extensions (thanks, Henry!).  Much of the description of regular
expressions below is copied verbatim from his manual entry.
.PP
An ARE is one or more \fIbranches\fR,
separated by `|'.
It matches anything that matches any of the branches.
.PP
A branch is zero or more \fIitems\fR, concatenated.
It matches a match for the first, followed by a match for the second, etc;
an empty branch matches the empty string.
.PP
An item is an \fIatom\fR possibly followed
by a single
\fIquantifier\fR:
`*', `+', `?', or a \fIbound\fR.
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.
.PP
A \fIbound\fR 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 255 inclusive,
and if there are two of them, the first may not exceed the second.
An atom followed by a bound containing one integer \fIi\fR
and no comma matches
a sequence of exactly \fIi\fR matches of the atom.
An atom followed by a bound
containing one integer \fIi\fR and a comma matches
a sequence of \fIi\fR or more matches of the atom.
An atom followed by a bound
containing two integers \fIi\fR and \fIj\fR matches
a sequence of \fIi\fR through \fIj\fR (inclusive) matches of the atom.
.PP
A quantifier followed by `?' (that is, `*?', `+?', `??', `{...}?') is
a \fInon-greedy quantifier\fR, which matches the same possibilities,
but prefers the smallest number rather than the largest number
of matches.
.PP
An atom is a regular expression enclosed in `()' (matching a match for the
regular expression, with the match noted for possible reporting),
an empty set of `()' (matching an empty string,
again noted for possible reporting),
either form with the leading parenthesis replaced by `(?:'
(``non-capturing parentheses'',
as opposed to the plainer ``capturing'' form;
they have the same matching semantics but do no reporting),
a \fIbracket expression\fR (see next section),
`.' (matching any single character),
a \fIconstraint\fR (see below),
a `\e' followed by any non-alphanumeric character
(matching that character taken as an ordinary character,
e.g. `\e\e' matches a backslash character),
an \fIescape\fR (see later section) beginning with `\e' followed by an
alphanumeric character,
or a single character with no other significance (matching that character).
.PP
A \fIconstraint\fR matches an empty string when specific conditions
are met.
(Some more constraints are described later.)
`^' matches at the beginning of a line,
and `$' matches at the end of a line;
these constraints are also known as \fIanchors\fR.
A regular expression enclosed in `(?=' and `)'
(\fIpositive lookahead\fR) matches at any point
where a string matching that regular expression begins.
A regular expression enclosed in `(?!' and `)'
(\fInegative lookahead\fR) matches at any point
where a string \fInot\fR matching that regular expression begins.
Lookahead constraints may not contain back references (see later),
and all parentheses within them are treated as non-capturing.
A constraint may not be followed by a quantifier.
.PP
A `{' followed by a character other than a digit is an ordinary
character, not the beginning of a bound.
An RE may not end with an `\e'.
.SH "BRACKET EXPRESSIONS"
A \fIbracket expression\fR is a list of characters enclosed in `[]'.
It normally matches any single character from the list (but see below).
If the list begins with `^',
it matches any single character
(but see below) \fInot\fR from the rest of the list.
If two characters in the list are separated by `\-', this is shorthand
for the full \fIrange\fR of characters between those two (inclusive) in the
collating sequence,
e.g. `[0\-9]' in ASCII matches any decimal digit.
Two ranges may not share an
endpoint, e.g. `a\-c\-e'.
Ranges are very collating-sequence-dependent,
and portable programs should avoid relying on them.
.PP
To include a literal `]' in the list, make it the first character
(following a possible `^'),
or precede it with `\e'.
To include a literal `\-', make it the first or last character,
or the second endpoint of a range,
or precede it with `\e'.
To use a literal `\-' as the first endpoint of a range,
enclose it in `[.' and `.]' to make it a collating element (see below),
or precede it with `\e'.
With the exception of these, some combinations using `[' (see next
paragraphs), and escapes,
all other special characters lose their
special significance within a bracket expression.
.PP
Within a bracket expression, a collating element (a character,
a multi-character 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 in a locale which has
multi-character collating elements
can thus match more than one character.
Most insidiously, if `^' is used,
this can happen even if no collating 
elements appear in the bracket expression!
If the collating sequence includes a `ch' collating element,
then the RE `[[.ch.]]*c' matches the first five characters
of `chchcc',
and the RE `[^c]b' matches all of `chb'.
.PP
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'o^' are the members of an equivalence class,
then `[[=o=]]', `[[=\o'o^'=]]', and `[o\o'o^']' are all synonymous.
An equivalence class may not be an endpoint
of a range.
.PP
Within a bracket expression, the name of a \fIcharacter class\fR enclosed
in `[:' and `:]' stands for the list of all characters belonging to that
class.
Standard character class names are:
.PP
.RS
.nf
.ta 3c 6c 9c
alnum	digit	punct
alpha	graph	space
blank	lower	upper
cntrl	print	xdigit
.fi
.RE
.PP
These stand for the character classes defined in
\fIctype\fR(3).
A locale may provide others.
A character class may not be used as an endpoint of a range.
.PP
There are two special cases of bracket expressions:
the bracket expressions `[[:<:]]' and `[[:>:]]'
are constraints, matching empty strings at
the beginning and end of a word respectively.
A word is defined as a sequence of
word characters
which is neither preceded nor followed by
word characters.
A word character is an
\fIalnum\fR
character (as defined by
\fIctype\fR(3))
or an underscore.
This is an extension, and its use is deprecated;
users of AREs should use constraint escapes instead (see below).
.SH ESCAPES
Escapes, which begin with a `\e' followed by an alphanumeric character,
come in several varieties:
character entry, class shorthands, constraint, and back references.
A `\e' followed by an alphanumeric character but not constituting
a valid escape is illegal in AREs.
In EREs, there are no escapes:
outside a bracket expression,
a `\e' followed by an alphanumeric character merely stands for that
character as an ordinary character,
and inside a bracket expression, `\e' is an ordinary character.
(The latter is the one actual incompatibility between POSIX and AREs.)
.PP
Character-entry escapes exist to make it easier to specify
non-printing and otherwise inconvenient characters in REs.
The Standard C escapes
`\ea' (alert, aka bell),
`\eb' (backspace),
`\ef' (formfeed),
`\en' (newline),
`\er' (carriage return),
`\et' (horizontal tab), and
`\ev' (vertical tab)
are all available.
`\ee' represents the character whose collating-sequence name
is ``ESC'', or failing that, the character with octal value 033.
`\eE' represents `\e' (reducing the backslash doubling in some
applications where there are multiple levels of backslash processing).
.PP
`\ecX', where X is any character, represents the character whose
low-order 5 bits are the same as those of X,
and whose other bits are all zero.
`\e0', `\eXY' and `\eXYZ', where X, Y, and Z are octal digits (0-7),
represent the characters whose octal values are 0, 0XY, and 0XYZ 
respectively.
(See below for why `\eX' isn't in the list here.)
`\exHHH', where HHH is any sequence of hexadecimal digits (0-9, A-F, a-f),
represents the character whose hexadecimal value is 0xHHH
(a single character no matter how many hexadecimal digits are used).
`\euWXYZ', where WXYZ is exactly four hexadecimal digits,
represents the Unicode character U+WXYZ in the local byte ordering.
`\eUstuvwxyz', where stuvwxyz is exactly eight hexadecimal digits,
is reserved for a somewhat-hypothetical Unicode extension to 32 bits.
.PP
The character-entry escapes are always taken as ordinary characters.
For example, `\e135' is `]' in ASCII,
but `\e135' does not terminate a bracket expression.
Beware, however, that some applications \- e.g., C compilers\-interpret 
such sequences themselves before the regular-expression package
gets to see them.
.PP
Class-shorthand escapes provide shorthands for certain commonly-used
character classes.
`\ed', `\es', and `\ew', outside bracket expressions, represent
`[[:digit:]]', `[[:space:]]', and `[[:alnum:]_]' respectively
(note the underscore in `\ew').
`\eD', `\eS', and `\eW', outside bracket expressions, represent
the complementary classes:
`[^[:digit:]]', `[^[:space:]]', and `[^[:alnum:]_]' respectively.
Within bracket expressions,
`\ed', `\es', and `\ew' lose their outer brackets, representing
`[:digit:]', `[:space:]', and `[:alnum:]_' respectively;
`\eD', `\eS', and `\eW' are illegal within bracket expressions.
.PP
A constraint escape is a constraint,
matching the empty string if specific conditions are met,
written as an escape.
`\eA' matches only at the beginning of the string, and
`\eZ' matches only at the end of the string
(see MATCHING, below, for how they differ from `^' and `$').
`\ey' matches only at the beginning or end of a word, and
`\eY' matches only at a point which is not the beginning or end of a word.
A word is defined as in the specification of [[:<:]] and [[:>:]] above.
Constraint escapes are illegal within bracket expressions.
.PP
A back reference is of the form `\eM', where M is a nonzero digit (1-9),
or `\eMNN', where MNN is some number of
digits (first 1-9, rest 0-9) and the value MNN
is not greater than the number of closing capturing parentheses seen so far.
A back reference matches the same string matched by the parenthesized
subexpression specified by the number,
so that (e.g.) `([bc])\e1' matches `bb' or `cc' but not `bc'.
The subexpression must entirely precede the back reference in the RE.
Subexpressions are numbered in the order of their leading parentheses.
Non-capturing parentheses do not define subexpressions.
.PP
There is an inherent historical ambiguity between octal character-entry 
escapes and back references, which is resolved by heuristics.
A single non-zero digit, not followed by another digit,
is always taken as a back reference.
A leading zero always indicates an octal escape.
A multi-digit sequence not starting with a zero is taken as a back 
reference if it comes after a suitable subexpression
(i.e. the number is in the legal range for a back reference),
and otherwise is taken as octal.
.SH "METASYNTAX"
In addition to the main syntax described above, there are some special
forms and miscellaneous syntactic facilities available.
These are extensions, compatible with but not specified by POSIX.
.PP
Normally the flavor of RE being used is specified by
application-dependent means.
However, this can be overridden by a \fIdirector\fR.
If an RE of any flavor begins with `***:', the rest of the RE is an ARE.
If an RE of any flavor begins with `***=',
the rest of the RE is taken to be a literal string,
with all characters considered ordinary characters.
.PP
An ARE may begin with \fIembedded options\fR:
the sequence `(?xyz)', where xyz is one or more alphabetic characters,
specifies options affecting the rest of the RE.
These supplement, and can override,
options specified by the application in application-specific ways.
The available option letters are:
.RS
.IP b 3
rest of RE is a BRE
.IP c
case-sensitive matching (usual default)
.IP e
rest of RE is an ERE
.IP i
case-insensitive matching (see MATCHING, below)
.IP m
historical synonym for `n'
.IP n
newline-sensitive matching (see MATCHING, below)
.IP p
partial newline-sensitive matching (see MATCHING, below)
.IP q
rest of RE is a literal (``quoted'') string, all ordinary characters
.IP s
non-newline-sensitive matching (usual default)
.IP t
tight syntax (usual default; see below)
.IP w
inverse partial newline-sensitive (``weird'') matching (see MATCHING, below)
.IP x
expanded syntax (see below)
.RE
.PP
In addition to the usual (\fItight\fR) RE syntax, in which all characters are
significant, there is an \fIexpanded\fR syntax,
available in all flavors of RE
by application-specified option, or in AREs by embedded x option.
In the expanded syntax,
any white-space character
not preceded by `\e'
and not within a bracket expression is ignored,
so a complex RE can be paragraphed legibly.
There is one exception:  white space is not allowed within multi-character
symbols like the ARE `(?:' or the BRE `\e('.
White-space characters are blank, tab, newline, etc. \- any character
defined as \fIspace\fR by
\fIctype\fR(3).
In addition, all characters between a non-backslashed non-bracket-expression
`#' and the following newline (or the end of the RE) are ignored,
so comments can be inserted conveniently.
Exactly how a multi-line expanded-syntax RE can be entered by a user,
if at all, is application-specific;
this is primarily a programming facility.
.PP
Finally, in an ARE,
outside bracket expressions, the sequence `(?#ttt)',
where ttt is any text not containing a `)', is a comment,
completely ignored.
Again, this is not allowed between the characters of
multi-character symbols like `(?:'.
Such comments are more a historical artifact than a useful facility,
and their use is deprecated;
use the expanded syntax instead.
.PP
\fINone\fR of these metasyntax extensions are available if the application
(or an initial `***=' director)
has specified that the user's input be treated as a literal string
rather than as an RE.
.SH "CHOOSING AMONG ALTERNATIVE MATCHES"
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.
.PP
Match lengths are measured in characters, not collating elements.
An empty string is considered longer than no match at all.
For example,
`bb*' matches the three middle characters of `abbbc',
`(week|wee)(night|knights)' 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 an empty string.
.PP
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,
e.g. `x' becomes `[xX]'.
When it appears inside a bracket expression, all case counterparts
of it are added to the bracket expression, so that (e.g.) `[x]'
becomes `[xX]' and `[^x]' becomes `[^xX]'.
.PP
If newline-sensitive matching is specified,
and the string supplied for matching contains newlines,
`.' and complemented bracket expressions (`[^...]')
will never match the newline character,
and `^' and `$' will match the empty string after and before a newline
respectively, in addition to matching at beginning and end of string
respectively.
ARE `\eA' and `\eZ' continue to match beginning or end of string \fIonly\fR.
.PP
If partial newline-sensitive matching is specified,
this affects `.' and complemented bracket expressions as with
newline-sensitive matching, but not `^' and `$'.
.PP
If inverse partial newline-sensitive matching is specified,
this affects `^' and `$' as with
newline-sensitive matching,
but not `.' and complemented bracket expressions.
This isn't very useful but is provided for symmetry.

.SH BASIC REGULAR EXPRESSIONS
BREs differ from EREs in several respects.
`|', `+', and `?' are ordinary characters and there is no equivalent
for their functionality.
The delimiters for bounds are `\e{' and `\e}',
with `{' and `}' by themselves ordinary characters.
The parentheses for nested subexpressions are `\e(' and `\e)',
with `(' and `)' by themselves ordinary characters.
`^' is an ordinary character except at the beginning of the
RE or the beginning of a parenthesized subexpression,
`$' is an ordinary character except at the end of the
RE or the end of a parenthesized subexpression,
and `*' is an ordinary character if it appears at the beginning of the
RE or the beginning of a parenthesized subexpression
(after a possible leading `^').
Finally,
single-digit back references (but no other escapes) are available.

.SH "LIMITS AND BACKWARD COMPATIBILITY"
No particular limit is imposed on the length of REs or the number
paired parentheses, brackets, or braces.
.PP
In AREs, a `\e' inside [...] is an escape,
so a literal `\e' within `[]' must be written `\e\e'.
.PP
In AREs, some escapes mean special things in a bracket expression
(i.e. `\ed', `\es', `\ew') and others are illegal (i.e. `\eD', `\eS',
`\eW', `\eA', `\eZ').
.PP
In AREs, a `{' followed by a digit will not match those two characters
but will instead start a bound.  Such sequences should be rare, and
will often result in an error because following characters will not
look like a valid bound.
.PP
In AREs, a `\e' followed by an alphanumeric character is either an
escape or an error.  A bunch of new escapes were treated as literal
characters in old versions of Tcl.
.PP
The longest-leftmost match is found in AREs.  Old versions of Tcl
found the first-leftmost match.  This may affect some old REs which
were written in the expectation that the first match would be
reported.  The careful crafting of old REs to optimize the search
order for fast matching is obsolete.  AREs examine all possible
matches in parallel, and their performance is largely insensitive to
their complexity, but cases where the search order was exploited to
deliberately find a match which was \fInot\fR the longest will need
rewriting.
.VE
.SH KEYWORDS
match, regular expression, string