regexp - Regular Expression Functions for Strings

This module contains functions for regular expression matching and substitution.

match(String, RegExp) -> MatchRes

Types

String = RegExp = string()

MatchRes = {match, Start, Length} | nomatch | {error, errordesc()}

Start = Length = integer()

Finds the first, longest match of the regular expression RegExp in String. This function searches for the longest possible match and returns the first one found if there are several expressions of the same length. It returns as follows:

{match, Start, Length}: if the match succeeded. Start is the starting position of the match, and Length is the length of the matching string.

nomatch: if there were no matching characters.

{error, Error}: if there was an error in RegExp.

first_match(String, RegExp) -> MatchRes

Types

String = RegExp = string()

MatchRes = {match, Start, Length} | nomatch | {error, errordesc()}

Start = Length = integer()

Finds the first match of the regular expression RegExp in String. This call is usually faster than match and it is also a useful way to ascertain that a match exists. It returns as follows:

{match, Start, Length}: if the match succeeded. Start is the starting position of the match and Length is the length of the matching string.

nomatch: if there were no matching characters.

{error, Error}: if there was an error in RegExp.

matches(String, RegExp) -> MatchRes

Types

String = RegExp = string()

MatchRes = {match, Matches} | {error, errordesc()}

Matches = list()

Finds all non-overlapping matches of the expression RegExp in String. It returns as follows:

{match, Matches}: if the regular expression was correct. The list will be empty if there was no match. Each element in the list looks like {Start, Length}, where Start is the starting position of the match, and Length is the length of the matching string.

{error, Error}: if there was an error in RegExp.

sub(String, RegExp, New) -> SubRes

Types

String = RegExp = New = string()

SubRes = {ok, NewString, RepCount} | {error, errordesc()}

RepCount = integer()

Substitutes the first occurrence of a substring matching RegExp in String with the string New. A & in the string New is replaced by the matched substring of String. \& puts a literal & into the replacement string. It returns as follows:

{ok, NewString, RepCount}: if RegExp is correct. RepCount is the number of replacements which have been made (this will be either 0 or 1).

{error, Error}: if there is an error in RegExp.

gsub(String, RegExp, New) -> SubRes

Types

String = RegExp = New = string()

SubRes = {ok, NewString, RepCount} | {error, errordesc()}

RepCount = integer()

The same as sub, except that all non-overlapping occurrences of a substring matching RegExp in String are replaced by the string New. It returns:

{ok, NewString, RepCount}: if RegExp is correct. RepCount is the number of replacements which have been made.

{error, Error}: if there is an error in RegExp.

split(String, RegExp) -> SplitRes

Types

String = RegExp = string()

SubRes = {ok, FieldList} | {error, errordesc()}

Fieldlist = [string()]

String is split into fields (sub-strings) by the regular expression RegExp.

If the separator expression is " " (a single space), then the fields are separated by blanks and/or tabs and leading and trailing blanks and tabs are discarded. For all other values of the separator, leading and trailing blanks and tabs are not discarded. It returns:

{ok, FieldList}: to indicate that the string has been split up into the fields of FieldList.

{error, Error}: if there is an error in RegExp.

sh_to_awk(ShRegExp) -> AwkRegExp

Types

ShRegExp AwkRegExp = string()

SubRes = {ok, NewString, RepCount} | {error, errordesc()}

RepCount = integer()

Converts the sh type regular expression ShRegExp into a full AWK regular expression. Returns the converted regular expression string. sh expressions are used in the shell for matching file names and have the following special characters:

*: matches any string including the null string.

?: matches any single character.

[...]: matches any of the enclosed characters. Character ranges are specified by a pair of characters separated by a -. If the first character after [ is a !, then any character not enclosed is matched.

It may sometimes be more practical to use sh type expansions as they are simpler and easier to use, even though they are not as powerful.

parse(RegExp) -> ParseRes

Types

RegExp = string()

ParseRes = {ok, RE} | {error, errordesc()}

Parses the regular expression RegExp and builds the internal representation used in the other regular expression functions. Such representations can be used in all of the other functions instead of a regular expression string. This is more efficient when the same regular expression is used in many strings. It returns:

{ok, RE} if RegExp is correct and RE is the internal representation.:

{error, Error} if there is an error in RegExpString.:

format_error(ErrorDescriptor) -> string()

Types

ErrorDescriptor = errordesc()

Returns a string which describes the error ErrorDescriptor returned when there is an error in a regular expression.

The regular expressions allowed here is a subset of the set found in egrep and in the AWK programming language, as defined in the book, The AWK Programming Language, by A. V. Aho, B. W. Kernighan, P. J. Weinberger. They are composed of the following characters:

c: matches the non-metacharacter c.

\c: matches the escape sequence or literal character c.

.: matches any character.

^: matches the beginning of a string.

$: matches the end of a string.

[abc...]: character class, which matches any of the characters abc... Character ranges are specified by a pair of characters separated by a -.

[^abc...]: negated character class, which matches any character except abc....

r1 | r2: alternation. It matches either r1 or r2.

r1r2: concatenation. It matches r1 and then r2.

r+: matches one or more rs.

r*: matches zero or more rs.

r?: matches zero or one rs.

(r): grouping. It matches r.

The escape sequences allowed are the same as for Erlang strings:

\b: backspace

\f: form feed

\n: newline (line feed)

\r: carriage return

\t: tab

\e: escape

\v: vertical tab

\s: space

\d: delete

\ddd: the octal value ddd

\c: any other character literally, for example \\ for backslash, \" for ")

To make these functions easier to use, in combination with the function io:get_line which terminates the input line with a new line, the $ characters also matches a string ending with "...\n". The following examples define Erlang data types:

Atoms     [a-z][0-9a-zA-Z_]*

Variables [A-Z_][0-9a-zA-Z_]*

Floats (\+|-)?[0-9]+\.[0-9]+((E|e)(\+|-)?[0-9]+)?

Regular expressions are written as Erlang strings when used with the functions in this module. This means that any \ or " characters in a regular expression string must be written with \ as they are also escape characters for the string. For example, the regular expression string for Erlang floats is:"(\\+|-)?[0-9]+\\.[0-9]+((E|e)(\\+|-)?[0-9]+)?".

It is not really necessary to have the escape sequences as part of the regular expression syntax as they can always be generated directly in the string. They are included for completeness and can they can also be useful when generating regular expressions, or when they are entered other than with Erlang strings.

Robert Virding - support@erlang.ericsson.se