# regexp - Match a regular expression against a string

## SYNOPSIS

regexp ?switches? exp string ?matchVar? ?subMatchVar subMatchVar ...?

## DESCRIPTION

Determines whether the regular expression exp matches part or all of string and returns 1 if it does, 0 if it doesn't.

If additional arguments are specified after string then they are treated as the names of variables in which to return information about which part(s) of string matched exp. MatchVar will be set to the range of string that matched all of exp. The first subMatchVar will contain the characters in string that matched the leftmost parenthesized subexpression within exp, the next subMatchVar will contain the characters that matched 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 upper-case characters in string to be treated as lower case during the matching process.

-indices
  Changes what is stored in the subMatchVars. Instead of storing the matching characters from string, each variable will contain 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 will be 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 doesn't match the string (e.g. because it was in a portion of the expression that wasn't matched), then the corresponding subMatchVar will be set to ``-1 -1'' if -indices has been specified or to an empty string otherwise.

## REGULAR EXPRESSIONS

Regular expressions are implemented using Henry Spencer's package (thanks, Henry!), and much of the description of regular expressions below is copied verbatim from his manual entry.

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, etc.

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 (e.g. ``[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:

[1]
If a regular expression could match two different parts of an input string then it will match the one that begins earliest.

[2]
If a regular expression contains | operators then the leftmost matching sub-expression is chosen.

[3]
In *, +, and ? constructs, longer matches are chosen in preference to shorter ones.

[4]
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) gets first shot at the input string 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.

## KEYWORDS

match, regular expression, string