regexp(TCL)

regexp -- match a regular expression against a string

Synopsis

regexp [switches] exp string [matchVar] [subMatchVar subMatchVar ...]

Description

regexp determines whether the regular expression exp matches part or all of string and returns 1 if it does, 0 if it does not.

If additional arguments are specified after string then they are treated as the names of variables in which to return information about which parts of string match exp. matchVar is set to the range of string that matched all of exp. The first subMatchVar contains the characters in string that match the leftmost parenthesized subexpression within exp, the next subMatchVar contains the characters that match the next parenthesized subexpression to the right in exp, and so on.

If the initial arguments to regexp start with ``-'' then they are treated as switches. The following switches are currently supported:

-nocase: Causes uppercase characters in string to be treated as lowercase during the matching process.
-indices: Changes what is stored in the subMatchVars. Instead of storing the matching characters from string, each variable contains a list of two decimal strings giving the indices in string of the first and last characters in the matching range of characters.
--: Marks the end of switches. The argument following this one is treated as exp even if it starts with a ``-''.

If there are more subMatchVar's than parenthesized subexpressions within exp, or if a particular subexpression in exp does not match the string (for example, because it was in a portion of the expression that was not matched), then the corresponding subMatchVar is set to -1 -1 if -indices has been specified, or to an empty string otherwise.

Regular expressions

Regular expressions are implemented using the public domain Henry Spencer package; the standard SCO^® regular expression syntax (see regexp(5)) does not apply. The following regular expressions are recognized by Tcl:

A regular expression is zero or more branches, separated by ``|''. It matches anything that matches one of the branches.

A branch is zero 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 ``'', ``+'', or ``?''. 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 match of the atom, or the null string.

An atom is a regular expression in parentheses (matching a match for the regular expression), a range (see below), ``.'' (matching any single character), ``^'' (matching the null string at the beginning of the input string), ``$'' (matching the null string at the end of the input string), a ``\'' followed by a single character (matching that character), or a single character with no other significance (matching that character).

A range is a sequence of characters enclosed in ``[]''. It normally matches any single character from the sequence. If the sequence begins with ``^'', it matches any single character not from the rest of the sequence. If two characters in the sequence are separated by ``-'', this is shorthand for the full list of ASCII characters between them (for example,``[0-9]'' matches any decimal digit). To include a literal ``]'' in the sequence, make it the first character (following a possible ``^''). To include a literal ``-'', make it the first or last character.

Choosing among alternative matches

In general there may be more than one way to match a regular expression to an input string. For example, consider the command:

regexp (a)b aabaaabb x y

Considering only the rules given so far, x and y could end up with the values aabb and aa, aaab and aaa, ab and a, or any of several other combinations. To resolve this potential ambiguity regexp chooses among alternatives using the rule ``first then longest''. In other words, it considers the possible matches in order, working from left to right across the input string and the pattern, and it attempts to match longer pieces of the input string before shorter ones. More specifically, the following rules apply in decreasing order of priority:

If a regular expression can match two different parts of an input string then it matches the one that begins earliest.
If a regular expression contains | operators then the leftmost matching sub-expression is chosen.
In , +, and ? constructs, longer matches are chosen in preference to shorter ones.
In sequences of expression components the components are considered from left to right.

In the example from above, (a)b matches aab: the (a*) portion of the pattern is matched first and it consumes the leading aa; then the b portion of the pattern consumes the next b. Or, consider the following example:

regexp (ab|a)(b)c abc x y z

After this command x will be abc, y will be ab, and z will be an empty string. Rule 4 specifies that (ab|a) is checked for a match against the input string first, and Rule 2 specifies that the ab sub-expression is checked before the a sub-expression. Thus the b has already been claimed before the (b) component is checked and (b) must match an empty string.

References

regexp(5)

Standards compliance

The regular expression package used in Tcl is not part of any currently supported standard; it was developed at the University of Toronto by Dr Henry Spencer and incorporated into Tcl at the University of California at Berkeley by Dr John Ousterhout, and is used by permission.