\section{Built-in Module \sectcode{re}} \label{module-re} \bimodindex{re} % XXX Remove before 1.5final release. {\large\bf The \code{re} module is still in the process of being developed, and more features will be added in future 1.5 alphas and betas. This documentation is also preliminary and incomplete. If you find a bug or documentation error, or just find something unclear, please send a message to \code{string-sig@python.org}, and we'll fix it.} This module provides regular expression matching operations similar to those found in Perl. It's 8-bit clean: both patterns and strings may contain null bytes and characters whose high bit is set. It is always available. Regular expressions use the backslash character (\code{\e}) to indicate special forms or to allow special characters to be used without invoking their special meaning. This collides with Python's usage of the same character for the same purpose in string literals; for example, to match a literal backslash, one might have to write \code{\e\e\e\e} as the pattern string, because the regular expression must be \code{\e\e}, and each backslash must be expressed as \code{\e\e} inside a regular Python string literal. The solution is to use Python's raw string notation for regular expression patterns; backslashes are not handled in any special way in a string literal prefixed with 'r'. So \code{r"\e n"} is a two character string containing a backslash and the letter 'n', while \code{"\e n"} is a one-character string containing a newline. Usually patterns will be expressed in Python code using this raw string notation. % XXX Can the following section be dropped, or should it be boiled down? %\strong{Please note:} There is a little-known fact about Python string %literals which means that you don't usually have to worry about %doubling backslashes, even though they are used to escape special %characters in string literals as well as in regular expressions. This %is because Python doesn't remove backslashes from string literals if %they are followed by an unrecognized escape character. %\emph{However}, if you want to include a literal \dfn{backslash} in a %regular expression represented as a string literal, you have to %\emph{quadruple} it or enclose it in a singleton character class. %E.g.\ to extract \LaTeX\ \code{\e section\{{\rm %\ldots}\}} headers from a document, you can use this pattern: %\code{'[\e ] section\{\e (.*\e )\}'}. \emph{Another exception:} %the escape sequence \code{\e b} is significant in string literals %(where it means the ASCII bell character) as well as in Emacs regular %expressions (where it stands for a word boundary), so in order to %search for a word boundary, you should use the pattern \code{'\e \e b'}. %Similarly, a backslash followed by a digit 0-7 should be doubled to %avoid interpretation as an octal escape. \subsection{Regular Expressions} A regular expression (or RE) specifies a set of strings that matches it; the functions in this module let you check if a particular string matches a given regular expression (or if a given regular expression matches a particular string, which comes down to the same thing). Regular expressions can be concatenated to form new regular expressions; if \emph{A} and \emph{B} are both regular expressions, then \emph{AB} is also an regular expression. If a string \emph{p} matches A and another string \emph{q} matches B, the string \emph{pq} will match AB. Thus, complex expressions can easily be constructed from simpler primitive expressions like the ones described here. For details of the theory and implementation of regular expressions, consult the Friedl book referenced below, or almost any textbook about compiler construction. A brief explanation of the format of regular expressions follows. For further information and a gentler presentation, consult XXX somewhere. Regular expressions can contain both special and ordinary characters. Most ordinary characters, like '\code{A}', '\code{a}', or '\code{0}', are the simplest regular expressions; they simply match themselves. You can concatenate ordinary characters, so '\code{last}' matches the characters 'last'. (In the rest of this section, we'll write RE's in \code{this special font}, usually without quotes, and strings to be matched 'in single quotes'.) Some characters, like \code{|} or \code{(}, are special. Special characters either stand for classes of ordinary characters, or affect how the regular expressions around them are interpreted. The special characters are: \begin{itemize} \item[\code{.}] (Dot.) In the default mode, this matches any character except a newline. If the \code{DOTALL} flag has been specified, this matches any character including a newline. \item[\code{\^}] (Caret.) Matches the start of the string, and in \code{MULTILINE} mode also immediately after each newline. \item[\code{\$}] Matches the end of the string. \code{foo} matches both 'foo' and 'foobar', while the regular expression '\code{foo\$}' matches only 'foo'. % \item[\code{*}] Causes the resulting RE to match 0 or more repetitions of the preceding RE, as many repetitions as are possible. \code{ab*} will match 'a', 'ab', or 'a' followed by any number of 'b's. % \item[\code{+}] Causes the resulting RE to match 1 or more repetitions of the preceding RE. \code{ab+} will match 'a' followed by any non-zero number of 'b's; it will not match just 'a'. % \item[\code{?}] Causes the resulting RE to match 0 or 1 repetitions of the preceding RE. \code{ab?} will match either 'a' or 'ab'. \item[\code{*?}, \code{+?}, \code{??}] The \code{*}, \code{+}, and \code{?} qualifiers are all \dfn{greedy}; they match as much text as possible. Sometimes this behaviour isn't desired; if the RE \code{<.*>} is matched against \code{

title

}, it will match the entire string, and not just \code{

}. Adding \code{?} after the qualifier makes it perform the match in \dfn{non-greedy} or \dfn{minimal} fashion; as few characters as possible will be matched. Using \code{.*?} in the previous expression, it will match only \code{

}. % \item[\code{\e}] Either escapes special characters (permitting you to match characters like '*?+\&\$'), or signals a special sequence; special sequences are discussed below. If you're not using a raw string to express the pattern, remember that Python also uses the backslash as an escape sequence in string literals; if the escape sequence isn't recognized by Python's parser, the backslash and subsequent character are included in the resulting string. However, if Python would recognize the resulting sequence, the backslash should be repeated twice. This is complicated and hard to understand, so it's highly recommended that you use raw strings. % \item[\code{[]}] Used to indicate a set of characters. Characters can be listed individually, or a range is indicated by giving two characters and separating them by a '-'. Special characters are not active inside sets. For example, \code{[akm\$]} will match any of the characters 'a', 'k', 'm', or '\$'; \code{[a-z]} will match any lowercase letter and \code{[a-zA-Z0-9]} matches any letter or digit. Character classes of the form \code{\e \var{X}} defined below are also acceptable. If you want to include a \code{]} or a \code{-} inside a set, precede it with a backslash. Characters \emph{not} within a range can be matched by including a \code{\^} as the first character of the set; \code{\^} elsewhere will simply match the '\code{\^}' character. % \item[\code{|}]\code{A|B}, where A and B can be arbitrary REs, creates a regular expression that will match either A or B. This can be used inside groups (see below) as well. To match a literal '|', use \code{\e|}, or enclose it inside a character class, like \code{[|]}. % \item[\code{( ... )}] Matches whatever regular expression is inside the parentheses, and indicates the start and end of a group; the contents of a group can be retrieved after a match has been performed, and can be matched later in the string with the \code{\e \var{number}} special sequence, described below. To match the literals '(' or ')', use \code{\e(} or \code{\e)}, or enclose them inside a character class: \code{[(] [)]}. % \item[\code{(?:...)}] A non-grouping version of regular parentheses. Matches whatever's inside the parentheses, but the text matched by the group \emph{cannot} be retrieved after performing a match or referenced later in the pattern. % \item[\code{(?P<\var{name}>...)}] Similar to regular parentheses, but the text matched by the group is accessible via the symbolic group name \var{name}. Group names must be valid Python identifiers. A symbolic group is also a numbered group, just as if the group were not named. So the group named 'id' in the example above can also be referenced as the numbered group 1. For example, if the pattern string is \code{r'(?P[a-zA-Z_]\e w*)'}, the group can be referenced by its name in arguments to methods of match objects, such as \code{m.group('id')} or \code{m.end('id')}, and also by name in pattern text (e.g. \code{(?P=id)}) and replacement text (e.g. \code{\e g}). % \item[\code{(?\#...)}] A comment; the contents of the parentheses are simply ignored. % \item[\code{(?=...)}] Matches if \code{RE} matches next. This is not implemented as of Python 1.5a3. % \item[\code{(?!...)}] Matches if \code{...} doesn't match next. This is not implemented as of Python 1.5a3. \end{itemize} The special sequences consist of '\code{\e}' and a character from the list below. If the ordinary character is not on the list, then the resulting RE will match the second character. For example, \code{\e\$} matches the character '\$'. Ones where the backslash should be doubled are indicated. \begin{itemize} % \item[\code{\e \var{number}}] Matches the contents of the group of the same number. For example, \code{(.+) \e 1} matches 'the the' or '55 55', but not 'the end' (note the space after the group). This special sequence can only be used to match one of the first 99 groups. If the first digit of \var{number} is 0, or \var{number} is 3 octal digits long, it will not interpreted as a group match, but as the character with octal value \var{number}. % \item[\code{\e A}] Matches only at the start of the string. % \item[\code{\e b}] Matches the empty string, but only at the beginning or end of a word. A word is defined as a sequence of alphanumeric characters, so the end of a word is indicated by whitespace or a non-alphanumeric character. % \item[\code{\e B}] Matches the empty string, but only when it is \emph{not} at the beginning or end of a word. % \item[\code{\e d}]Matches any decimal digit; this is equivalent to the set \code{[0-9]}. % \item[\code{\e D}]Matches any non-digit character; this is equivalent to the set \code{[\^0-9]}. % \item[\code{\e s}]Matches any whitespace character; this is equivalent to the set \code{[ \e t\e n\e r\e f\e v]}. % \item[\code{\e S}]Matches any non-whitespace character; this is equivalent to the set \code{[\^ \e t\e n\e r\e f\e v]}. % \item[\code{\e w}]Matches any alphanumeric character; this is equivalent to the set \code{[a-zA-Z0-9_]}. % \item[\code{\e W}] Matches any non-alphanumeric character; this is equivalent to the set \code{[\^ a-zA-Z0-9_]}. \item[\code{\e Z}]Matches only at the end of the string. % \item[\code{\e \e}] Matches a literal backslash. \end{itemize} \subsection{Module Contents} The module defines the following functions and constants, and an exception: \renewcommand{\indexsubitem}{(in module re)} \begin{funcdesc}{compile}{pattern\optional{\, flags}} Compile a regular expression pattern into a regular expression object, which can be used for matching using its \code{match} and \code{search} methods, described below. The sequence % \bcode\begin{verbatim} prog = re.compile(pat) result = prog.match(str) \end{verbatim}\ecode % is equivalent to % \bcode\begin{verbatim} result = re.match(pat, str) \end{verbatim}\ecode % but the version using \code{compile()} is more efficient when multiple regular expressions are used concurrently in a single program. %(The compiled version of the last pattern passed to \code{regex.match()} or %\code{regex.search()} is cached, so programs that use only a single %regular expression at a time needn't worry about compiling regular %expressions.) \end{funcdesc} \begin{funcdesc}{escape}{string} Return \var{string} with all non-alphanumerics backslashed; this is useful if you want to match some variable string which may have regular expression metacharacters in it. \end{funcdesc} \begin{funcdesc}{match}{pattern\, string\optional{\, flags}} If zero or more characters at the beginning of \var{string} match the regular expression \var{pattern}, return a corresponding \code{Match} object. Return \code{None} if the string does not match the pattern; note that this is different from a zero-length match. \end{funcdesc} \begin{funcdesc}{search}{pattern\, string\optional{\, flags}} Scan through \var{string} looking for a location where the regular expression \var{pattern} produces a match. Return \code{None} if no position in the string matches the pattern; note that this is different from finding a zero-length match at some point in the string. \end{funcdesc} \begin{funcdesc}{split}{pattern\, string\, \optional{, maxsplit=0}} Split \var{string} by the occurrences of \var{pattern}. If capturing parentheses are used in pattern, then occurrences of patterns or subpatterns are also returned. % \bcode\begin{verbatim} >>> re.split('[\W]+', 'Words, words, words.') ['Words', 'words', 'words', ''] >>> re.split('([\W]+)', 'Words, words, words.') ['Words', ', ', 'words', ', ', 'words', '.', ''] \end{verbatim}\ecode % This function combines and extends the functionality of \code{regex.split()} and \code{regex.splitx()}. \end{funcdesc} \begin{funcdesc}{sub}{pattern\, repl\, string\optional{, count=0}} Return the string obtained by replacing the leftmost non-overlapping occurrences of \var{pattern} in \var{string} by the replacement \var{repl}, which can be a string or the function that returns a string. If the pattern isn't found, \var{string} is returned unchanged. The pattern may be a string or a regexp object; if you need to specify regular expression flags, you must use a regexp object, or use embedded modifiers in a pattern string; e.g. % \bcode\begin{verbatim} sub("(?i)b+", "x", "bbbb BBBB") returns 'x x'. \end{verbatim}\ecode % The optional argument \var{count} is the maximum number of pattern occurrences to be replaced; count must be a non-negative integer, and the default value of 0 means to replace all occurrences. Empty matches for the pattern are replaced only when not adjacent to a previous match, so \code{sub('x*', '-', 'abc')} returns '-a-b-c-'. \end{funcdesc} \begin{funcdesc}{subn}{pattern\, repl\, string\optional{, count=0}} Perform the same operation as \code{sub()}, but return a tuple \code{(new_string, number_of_subs_made)}. \end{funcdesc} \begin{excdesc}{error} Exception raised when a string passed to one of the functions here is not a valid regular expression (e.g., unmatched parentheses) or when some other error occurs during compilation or matching. (It is never an error if a string contains no match for a pattern.) \end{excdesc} \subsection{Regular Expression Objects} Compiled regular expression objects support the following methods and attributes: \renewcommand{\indexsubitem}{(re method)} \begin{funcdesc}{match}{string\optional{\, pos}} If zero or more characters at the beginning of \var{string} match this regular expression, return a corresponding \code{Match} object. Return \code{None} if the string does not match the pattern; note that this is different from a zero-length match. The optional second parameter \var{pos} gives an index in the string where the search is to start; it defaults to \code{0}. This is not completely equivalent to slicing the string; the \code{'\^'} pattern character matches at the real begin of the string and at positions just after a newline, not necessarily at the index where the search is to start. \end{funcdesc} \begin{funcdesc}{search}{string\optional{\, pos}} Scan through \var{string} looking for a location where this regular expression produces a match. Return \code{None} if no position in the string matches the pattern; note that this is different from finding a zero-length match at some point in the string. The optional second parameter has the same meaning as for the \code{match} method. \end{funcdesc} \begin{funcdesc}{split}{string\, \optional{, maxsplit=0}} Identical to the \code{split} function, using the compiled pattern. \end{funcdesc} \begin{funcdesc}{sub}{repl\, string\optional{, count=0}} Identical to the \code{sub} function, using the compiled pattern. \end{funcdesc} \begin{funcdesc}{subn}{repl\, string\optional{, count=0}} Identical to the \code{subn} function, using the compiled pattern. \end{funcdesc} \renewcommand{\indexsubitem}{(regex attribute)} \begin{datadesc}{flags} The flags argument used when the regex object was compiled, or 0 if no flags were provided. \end{datadesc} \begin{datadesc}{groupindex} A dictionary mapping any symbolic group names (defined by \code{?P<\var{id}>}) to group numbers. The dictionary is empty if no symbolic groups were used in the pattern. \end{datadesc} \begin{datadesc}{pattern} The pattern string from which the regex object was compiled. \end{datadesc} \subsection{Match Objects} Match objects support the following methods and attributes: \begin{funcdesc}{span}{group} Return the 2-tuple \code{(start(\var{group}), end(\var{group}))}. Note that if \var{group} did not contribute to the match, this is \code{(None, None)}. \end{funcdesc} \begin{funcdesc}{start}{group} \end{funcdesc} \begin{funcdesc}{end}{group} Return the indices of the start and end of the substring matched by \var{group}. Return \code{None} if \var{group} exists but did not contribute to the match. Note that for a match object \code{m}, and a group \code{g} that did contribute to the match, the substring matched by group \code{g} is \bcode\begin{verbatim} m.string[m.start(g):m.end(g)] \end{verbatim}\ecode % Note too that \code{m.start(\var{group})} will equal \code{m.end(\var{group})} if \var{group} matched a null string. For example, after \code{m = re.search('b(c?)', 'cba')}, \code{m.start(0)} is 1, \code{m.end(0)} is 2, \code{m.start(1)} and \code{m.end(1)} are both 2, and \code{m.start(2)} raises an \code{IndexError} exception. \end{funcdesc} \begin{funcdesc}{group}{\optional{g1, g2, ...})} This method is only valid when the last call to the \code{match} or \code{search} method found a match. It returns one or more groups of the match. If there is a single \var{index} argument, the result is a single string; if there are multiple arguments, the result is a tuple with one item per argument. If the \var{index} is zero, the corresponding return value is the entire matching string; if it is in the inclusive range [1..99], it is the string matching the the corresponding parenthesized group (using the default syntax, groups are parenthesized using \code{\e (} and \code{\e )}). If no such group exists, the corresponding result is \code{None}. If the regular expression was compiled by \code{symcomp} instead of \code{compile}, the \var{index} arguments may also be strings identifying groups by their group name. \end{funcdesc} \begin{datadesc}{pos} The index at which the search or match began. \end{datadesc} \begin{datadesc}{re} The regular expression object whose match() or search() method produced this match object. \end{datadesc} \begin{datadesc}{string} The string passed to \code{match()} or \code{search()}. \end{datadesc} \begin{seealso} \seetext Jeffrey Friedl, \emph{Mastering Regular Expressions}. \end{seealso}