\section{\module{re} --- Regular expression operations} \declaremodule{standard}{re} \moduleauthor{Fredrik Lundh}{fredrik@pythonware.com} \sectionauthor{Andrew M. Kuchling}{amk@amk.ca} \modulesynopsis{Regular expression search and match operations with a Perl-style expression syntax.} This module provides regular expression matching operations similar to those found in Perl. Regular expression pattern strings may not contain null bytes, but can specify the null byte using the \code{\e\var{number}} notation. Both patterns and strings to be searched can be Unicode strings as well as 8-bit strings. The \module{re} module is always available. Regular expressions use the backslash character (\character{\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 \samp{\e\e}, and each backslash must be expressed as \samp{\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 \character{r}. So \code{r"\e n"} is a two-character string containing \character{\e} and \character{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. \begin{seealso} \seetitle{Mastering Regular Expressions}{Book on regular expressions by Jeffrey Friedl, published by O'Reilly. The Python material in this book dates from before the \refmodule{re} module, but it covers writing good regular expression patterns in great detail.} \end{seealso} \subsection{Regular Expression Syntax \label{re-syntax}} 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 a regular expression. If a string \emph{p} matches A and another string \emph{q} matches B, the string \emph{pq} will match AB if \emph{A} and \emph{B} do no specify boundary conditions that are no longer satisfied by \emph{pq}. 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 the Regular Expression HOWTO, accessible from \url{http://www.python.org/doc/howto/}. Regular expressions can contain both special and ordinary characters. Most ordinary characters, like \character{A}, \character{a}, or \character{0}, are the simplest regular expressions; they simply match themselves. You can concatenate ordinary characters, so \regexp{last} matches the string \code{'last'}. (In the rest of this section, we'll write RE's in \regexp{this special style}, usually without quotes, and strings to be matched \code{'in single quotes'}.) Some characters, like \character{|} or \character{(}, 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{list}{}{\leftmargin 0.7in \labelwidth 0.65in} \item[\character{.}] (Dot.) In the default mode, this matches any character except a newline. If the \constant{DOTALL} flag has been specified, this matches any character including a newline. \item[\character{\textasciicircum}] (Caret.) Matches the start of the string, and in \constant{MULTILINE} mode also matches immediately after each newline. \item[\character{\$}] Matches the end of the string or just before the newline at the end of the string, and in \constant{MULTILINE} mode also matches before a newline. \regexp{foo} matches both 'foo' and 'foobar', while the regular expression \regexp{foo\$} matches only 'foo'. More interestingly, searching for \regexp{foo.\$} in 'foo1\textbackslash nfoo2\textbackslash n' matches 'foo2' normally, but 'foo1' in \constant{MULTILINE} mode. \item[\character{*}] Causes the resulting RE to match 0 or more repetitions of the preceding RE, as many repetitions as are possible. \regexp{ab*} will match 'a', 'ab', or 'a' followed by any number of 'b's. \item[\character{+}] Causes the resulting RE to match 1 or more repetitions of the preceding RE. \regexp{ab+} will match 'a' followed by any non-zero number of 'b's; it will not match just 'a'. \item[\character{?}] Causes the resulting RE to match 0 or 1 repetitions of the preceding RE. \regexp{ab?} will match either 'a' or 'ab'. \item[\code{*?}, \code{+?}, \code{??}] The \character{*}, \character{+}, and \character{?} qualifiers are all \dfn{greedy}; they match as much text as possible. Sometimes this behaviour isn't desired; if the RE \regexp{<.*>} is matched against \code{'

title

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

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

'}. \item[\code{\{\var{m}\}}] Specifies that exactly \var{m} copies of the previous RE should be matched; fewer matches cause the entire RE not to match. For example, \regexp{a\{6\}} will match exactly six \character{a} characters, but not five. \item[\code{\{\var{m},\var{n}\}}] Causes the resulting RE to match from \var{m} to \var{n} repetitions of the preceding RE, attempting to match as many repetitions as possible. For example, \regexp{a\{3,5\}} will match from 3 to 5 \character{a} characters. Omitting \var{n} specifies an infinite upper bound; you can't omit \var{m}. As an example, \regexp{a\{4,\}b} will match \code{aaaab}, a thousand \character{a} characters followed by a \code{b}, but not \code{aaab}. The comma may not be omitted or the modifier would be confused with the previously described form. \item[\code{\{\var{m},\var{n}\}?}] Causes the resulting RE to match from \var{m} to \var{n} repetitions of the preceding RE, attempting to match as \emph{few} repetitions as possible. This is the non-greedy version of the previous qualifier. For example, on the 6-character string \code{'aaaaaa'}, \regexp{a\{3,5\}} will match 5 \character{a} characters, while \regexp{a\{3,5\}?} will only match 3 characters. \item[\character{\e}] Either escapes special characters (permitting you to match characters like \character{*}, \character{?}, and so forth), 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 for all but the simplest expressions. \item[\code{[]}] Used to indicate a set of characters. Characters can be listed individually, or a range of characters can be indicated by giving two characters and separating them by a \character{-}. Special characters are not active inside sets. For example, \regexp{[akm\$]} will match any of the characters \character{a}, \character{k}, \character{m}, or \character{\$}; \regexp{[a-z]} will match any lowercase letter, and \code{[a-zA-Z0-9]} matches any letter or digit. Character classes such as \code{\e w} or \code{\e S} (defined below) are also acceptable inside a range. If you want to include a \character{]} or a \character{-} inside a set, precede it with a backslash, or place it as the first character. The pattern \regexp{[]]} will match \code{']'}, for example. You can match the characters not within a range by \dfn{complementing} the set. This is indicated by including a \character{\textasciicircum} as the first character of the set; \character{\textasciicircum} elsewhere will simply match the \character{\textasciicircum} character. For example, \regexp{[{\textasciicircum}5]} will match any character except \character{5}, and \regexp{[\textasciicircum\code{\textasciicircum}]} will match any character except \character{\textasciicircum}. \item[\character{|}]\code{A|B}, where A and B can be arbitrary REs, creates a regular expression that will match either A or B. An arbitrary number of REs can be separated by the \character{|} in this way. This can be used inside groups (see below) as well. REs separated by \character{|} are tried from left to right, and the first one that allows the complete pattern to match is considered the accepted branch. This means that if \code{A} matches, \code{B} will never be tested, even if it would produce a longer overall match. In other words, the \character{|} operator is never greedy. To match a literal \character{|}, use \regexp{\e|}, or enclose it inside a character class, as in \regexp{[|]}. \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 \regexp{\e \var{number}} special sequence, described below. To match the literals \character{(} or \character{)}, use \regexp{\e(} or \regexp{\e)}, or enclose them inside a character class: \regexp{[(] [)]}. \item[\code{(?...)}] This is an extension notation (a \character{?} following a \character{(} is not meaningful otherwise). The first character after the \character{?} determines what the meaning and further syntax of the construct is. Extensions usually do not create a new group; \regexp{(?P<\var{name}>...)} is the only exception to this rule. Following are the currently supported extensions. \item[\code{(?iLmsux)}] (One or more letters from the set \character{i}, \character{L}, \character{m}, \character{s}, \character{u}, \character{x}.) The group matches the empty string; the letters set the corresponding flags (\constant{re.I}, \constant{re.L}, \constant{re.M}, \constant{re.S}, \constant{re.U}, \constant{re.X}) for the entire regular expression. This is useful if you wish to include the flags as part of the regular expression, instead of passing a \var{flag} argument to the \function{compile()} function. Note that the \regexp{(?x)} flag changes how the expression is parsed. It should be used first in the expression string, or after one or more whitespace characters. If there are non-whitespace characters before the flag, the results are undefined. \item[\code{(?:...)}] A non-grouping version of regular parentheses. Matches whatever regular expression is inside the parentheses, but the substring 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 substring matched by the group is accessible via the symbolic group name \var{name}. Group names must be valid Python identifiers, and each group name must be defined only once within a regular expression. 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 is \regexp{(?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 (for example, \regexp{(?P=id)}) and replacement text (such as \code{\e g}). \item[\code{(?P=\var{name})}] Matches whatever text was matched by the earlier group named \var{name}. \item[\code{(?\#...)}] A comment; the contents of the parentheses are simply ignored. \item[\code{(?=...)}] Matches if \regexp{...} matches next, but doesn't consume any of the string. This is called a lookahead assertion. For example, \regexp{Isaac (?=Asimov)} will match \code{'Isaac~'} only if it's followed by \code{'Asimov'}. \item[\code{(?!...)}] Matches if \regexp{...} doesn't match next. This is a negative lookahead assertion. For example, \regexp{Isaac (?!Asimov)} will match \code{'Isaac~'} only if it's \emph{not} followed by \code{'Asimov'}. \item[\code{(?<=...)}] Matches if the current position in the string is preceded by a match for \regexp{...} that ends at the current position. This is called a \dfn{positive lookbehind assertion}. \regexp{(?<=abc)def} will find a match in \samp{abcdef}, since the lookbehind will back up 3 characters and check if the contained pattern matches. The contained pattern must only match strings of some fixed length, meaning that \regexp{abc} or \regexp{a|b} are allowed, but \regexp{a*} and \regexp{a\{3,4\}} are not. Note that patterns which start with positive lookbehind assertions will never match at the beginning of the string being searched; you will most likely want to use the \function{search()} function rather than the \function{match()} function: \begin{verbatim} >>> import re >>> m = re.search('(?<=abc)def', 'abcdef') >>> m.group(0) 'def' \end{verbatim} This example looks for a word following a hyphen: \begin{verbatim} >>> m = re.search('(?<=-)\w+', 'spam-egg') >>> m.group(0) 'egg' \end{verbatim} \item[\code{(?}.) Inside the \character{[} and \character{]} of a character class, all numeric escapes are treated as characters. \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 or underscore characters, so the end of a word is indicated by whitespace or a non-alphanumeric, non-underscore character. Note that {}\code{\e b} is defined as the boundary between \code{\e w} and \code{\e W}, so the precise set of characters deemed to be alphanumeric depends on the values of the \code{UNICODE} and \code{LOCALE} flags. Inside a character range, \regexp{\e b} represents the backspace character, for compatibility with Python's string literals. \item[\code{\e B}] Matches the empty string, but only when it is \emph{not} at the beginning or end of a word. This is just the opposite of {}\code{\e b}, so is also subject to the settings of \code{LOCALE} and \code{UNICODE}. \item[\code{\e d}]Matches any decimal digit; this is equivalent to the set \regexp{[0-9]}. \item[\code{\e D}]Matches any non-digit character; this is equivalent to the set \regexp{[{\textasciicircum}0-9]}. \item[\code{\e s}]Matches any whitespace character; this is equivalent to the set \regexp{[ \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 \regexp{[\textasciicircum\ \e t\e n\e r\e f\e v]}. \item[\code{\e w}]When the \constant{LOCALE} and \constant{UNICODE} flags are not specified, matches any alphanumeric character and the underscore; this is equivalent to the set \regexp{[a-zA-Z0-9_]}. With \constant{LOCALE}, it will match the set \regexp{[0-9_]} plus whatever characters are defined as alphanumeric for the current locale. If \constant{UNICODE} is set, this will match the characters \regexp{[0-9_]} plus whatever is classified as alphanumeric in the Unicode character properties database. \item[\code{\e W}]When the \constant{LOCALE} and \constant{UNICODE} flags are not specified, matches any non-alphanumeric character; this is equivalent to the set \regexp{[{\textasciicircum}a-zA-Z0-9_]}. With \constant{LOCALE}, it will match any character not in the set \regexp{[0-9_]}, and not defined as alphanumeric for the current locale. If \constant{UNICODE} is set, this will match anything other than \regexp{[0-9_]} and characters marked as alphanumeric in the Unicode character properties database. \item[\code{\e Z}]Matches only at the end of the string. \end{list} Most of the standard escapes supported by Python string literals are also accepted by the regular expression parser: \begin{verbatim} \a \b \f \n \r \t \v \x \\ \end{verbatim} Octal escapes are included in a limited form: If the first digit is a 0, or if there are three octal digits, it is considered an octal escape. Otherwise, it is a group reference. % Note the lack of a period in the section title; it causes problems % with readers of the GNU info version. See http://www.python.org/sf/581414. \subsection{Matching vs Searching \label{matching-searching}} \sectionauthor{Fred L. Drake, Jr.}{fdrake@acm.org} Python offers two different primitive operations based on regular expressions: match and search. If you are accustomed to Perl's semantics, the search operation is what you're looking for. See the \function{search()} function and corresponding method of compiled regular expression objects. Note that match may differ from search using a regular expression beginning with \character{\textasciicircum}: \character{\textasciicircum} matches only at the start of the string, or in \constant{MULTILINE} mode also immediately following a newline. The ``match'' operation succeeds only if the pattern matches at the start of the string regardless of mode, or at the starting position given by the optional \var{pos} argument regardless of whether a newline precedes it. % Examples from Tim Peters: \begin{verbatim} re.compile("a").match("ba", 1) # succeeds re.compile("^a").search("ba", 1) # fails; 'a' not at start re.compile("^a").search("\na", 1) # fails; 'a' not at start re.compile("^a", re.M).search("\na", 1) # succeeds re.compile("^a", re.M).search("ba", 1) # fails; no preceding \n \end{verbatim} \subsection{Module Contents} \nodename{Contents of Module re} The module defines the following functions and constants, and an exception: \begin{funcdesc}{compile}{pattern\optional{, flags}} Compile a regular expression pattern into a regular expression object, which can be used for matching using its \function{match()} and \function{search()} methods, described below. The expression's behaviour can be modified by specifying a \var{flags} value. Values can be any of the following variables, combined using bitwise OR (the \code{|} operator). The sequence \begin{verbatim} prog = re.compile(pat) result = prog.match(str) \end{verbatim} is equivalent to \begin{verbatim} result = re.match(pat, str) \end{verbatim} but the version using \function{compile()} is more efficient when the expression will be used several times in a single program. %(The compiled version of the last pattern passed to %\function{re.match()} or \function{re.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{datadesc}{I} \dataline{IGNORECASE} Perform case-insensitive matching; expressions like \regexp{[A-Z]} will match lowercase letters, too. This is not affected by the current locale. \end{datadesc} \begin{datadesc}{L} \dataline{LOCALE} Make \regexp{\e w}, \regexp{\e W}, \regexp{\e b}, and \regexp{\e B} dependent on the current locale. \end{datadesc} \begin{datadesc}{M} \dataline{MULTILINE} When specified, the pattern character \character{\textasciicircum} matches at the beginning of the string and at the beginning of each line (immediately following each newline); and the pattern character \character{\$} matches at the end of the string and at the end of each line (immediately preceding each newline). By default, \character{\textasciicircum} matches only at the beginning of the string, and \character{\$} only at the end of the string and immediately before the newline (if any) at the end of the string. \end{datadesc} \begin{datadesc}{S} \dataline{DOTALL} Make the \character{.} special character match any character at all, including a newline; without this flag, \character{.} will match anything \emph{except} a newline. \end{datadesc} \begin{datadesc}{U} \dataline{UNICODE} Make \regexp{\e w}, \regexp{\e W}, \regexp{\e b}, and \regexp{\e B} dependent on the Unicode character properties database. \versionadded{2.0} \end{datadesc} \begin{datadesc}{X} \dataline{VERBOSE} This flag allows you to write regular expressions that look nicer. Whitespace within the pattern is ignored, except when in a character class or preceded by an unescaped backslash, and, when a line contains a \character{\#} neither in a character class or preceded by an unescaped backslash, all characters from the leftmost such \character{\#} through the end of the line are ignored. % XXX should add an example here \end{datadesc} \begin{funcdesc}{search}{pattern, string\optional{, flags}} Scan through \var{string} looking for a location where the regular expression \var{pattern} produces a match, and return a corresponding \class{MatchObject} instance. 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}{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 \class{MatchObject} instance. Return \code{None} if the string does not match the pattern; note that this is different from a zero-length match. \note{If you want to locate a match anywhere in \var{string}, use \method{search()} instead.} \end{funcdesc} \begin{funcdesc}{split}{pattern, string\optional{, maxsplit\code{ = 0}}} Split \var{string} by the occurrences of \var{pattern}. If capturing parentheses are used in \var{pattern}, then the text of all groups in the pattern are also returned as part of the resulting list. If \var{maxsplit} is nonzero, at most \var{maxsplit} splits occur, and the remainder of the string is returned as the final element of the list. (Incompatibility note: in the original Python 1.5 release, \var{maxsplit} was ignored. This has been fixed in later releases.) \begin{verbatim} >>> re.split('\W+', 'Words, words, words.') ['Words', 'words', 'words', ''] >>> re.split('(\W+)', 'Words, words, words.') ['Words', ', ', 'words', ', ', 'words', '.', ''] >>> re.split('\W+', 'Words, words, words.', 1) ['Words', 'words, words.'] \end{verbatim} This function combines and extends the functionality of the old \function{regsub.split()} and \function{regsub.splitx()}. \end{funcdesc} \begin{funcdesc}{findall}{pattern, string} Return a list of all non-overlapping matches of \var{pattern} in \var{string}. If one or more groups are present in the pattern, return a list of groups; this will be a list of tuples if the pattern has more than one group. Empty matches are included in the result. \versionadded{1.5.2} \end{funcdesc} \begin{funcdesc}{finditer}{pattern, string} Return an iterator over all non-overlapping matches for the RE \var{pattern} in \var{string}. For each match, the iterator returns a match object. Empty matches are included in the result. \versionadded{2.2} \end{funcdesc} \begin{funcdesc}{sub}{pattern, repl, string\optional{, count}} Return the string obtained by replacing the leftmost non-overlapping occurrences of \var{pattern} in \var{string} by the replacement \var{repl}. If the pattern isn't found, \var{string} is returned unchanged. \var{repl} can be a string or a function; if it is a string, any backslash escapes in it are processed. That is, \samp{\e n} is converted to a single newline character, \samp{\e r} is converted to a linefeed, and so forth. Unknown escapes such as \samp{\e j} are left alone. Backreferences, such as \samp{\e6}, are replaced with the substring matched by group 6 in the pattern. For example: \begin{verbatim} >>> re.sub(r'def\s+([a-zA-Z_][a-zA-Z_0-9]*)\s*\(\s*\):', ... r'static PyObject*\npy_\1(void)\n{', ... 'def myfunc():') 'static PyObject*\npy_myfunc(void)\n{' \end{verbatim} If \var{repl} is a function, it is called for every non-overlapping occurrence of \var{pattern}. The function takes a single match object argument, and returns the replacement string. For example: \begin{verbatim} >>> def dashrepl(matchobj): .... if matchobj.group(0) == '-': return ' ' .... else: return '-' >>> re.sub('-{1,2}', dashrepl, 'pro----gram-files') 'pro--gram files' \end{verbatim} The pattern may be a string or an RE object; if you need to specify regular expression flags, you must use a RE object, or use embedded modifiers in a pattern; for example, \samp{sub("(?i)b+", "x", "bbbb BBBB")} returns \code{'x x'}. The optional argument \var{count} is the maximum number of pattern occurrences to be replaced; \var{count} must be a non-negative integer. If omitted or zero, all occurrences will be replaced. Empty matches for the pattern are replaced only when not adjacent to a previous match, so \samp{sub('x*', '-', 'abc')} returns \code{'-a-b-c-'}. In addition to character escapes and backreferences as described above, \samp{\e g} will use the substring matched by the group named \samp{name}, as defined by the \regexp{(?P...)} syntax. \samp{\e g} uses the corresponding group number; \samp{\e g<2>} is therefore equivalent to \samp{\e 2}, but isn't ambiguous in a replacement such as \samp{\e g<2>0}. \samp{\e 20} would be interpreted as a reference to group 20, not a reference to group 2 followed by the literal character \character{0}. The backreference \samp{\e g<0>} substitutes in the entire substring matched by the RE. \end{funcdesc} \begin{funcdesc}{subn}{pattern, repl, string\optional{, count}} Perform the same operation as \function{sub()}, but return a tuple \code{(\var{new_string}, \var{number_of_subs_made})}. \end{funcdesc} \begin{funcdesc}{escape}{string} Return \var{string} with all non-alphanumerics backslashed; this is useful if you want to match an arbitrary literal string that may have regular expression metacharacters in it. \end{funcdesc} \begin{excdesc}{error} Exception raised when a string passed to one of the functions here is not a valid regular expression (for example, it might contain 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 \label{re-objects}} Compiled regular expression objects support the following methods and attributes: \begin{methoddesc}[RegexObject]{search}{string\optional{, pos\optional{, endpos}}} Scan through \var{string} looking for a location where this regular expression produces a match, and return a corresponding \class{MatchObject} instance. 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 \var{pos} and \var{endpos} parameters have the same meaning as for the \method{match()} method. \end{methoddesc} \begin{methoddesc}[RegexObject]{match}{string\optional{, pos\optional{, endpos}}} If zero or more characters at the beginning of \var{string} match this regular expression, return a corresponding \class{MatchObject} instance. Return \code{None} if the string does not match the pattern; note that this is different from a zero-length match. \note{If you want to locate a match anywhere in \var{string}, use \method{search()} instead.} 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{'\textasciicircum'} pattern character matches at the real beginning of the string and at positions just after a newline, but not necessarily at the index where the search is to start. The optional parameter \var{endpos} limits how far the string will be searched; it will be as if the string is \var{endpos} characters long, so only the characters from \var{pos} to \code{\var{endpos} - 1} will be searched for a match. If \var{endpos} is less than \var{pos}, no match will be found, otherwise, if \var{rx} is a compiled regular expression object, \code{\var{rx}.match(\var{string}, 0, 50)} is equivalent to \code{\var{rx}.match(\var{string}[:50], 0)}. \end{methoddesc} \begin{methoddesc}[RegexObject]{split}{string\optional{, maxsplit\code{ = 0}}} Identical to the \function{split()} function, using the compiled pattern. \end{methoddesc} \begin{methoddesc}[RegexObject]{findall}{string} Identical to the \function{findall()} function, using the compiled pattern. \end{methoddesc} \begin{methoddesc}[RegexObject]{finditer}{string} Identical to the \function{finditer()} function, using the compiled pattern. \end{methoddesc} \begin{methoddesc}[RegexObject]{sub}{repl, string\optional{, count\code{ = 0}}} Identical to the \function{sub()} function, using the compiled pattern. \end{methoddesc} \begin{methoddesc}[RegexObject]{subn}{repl, string\optional{, count\code{ = 0}}} Identical to the \function{subn()} function, using the compiled pattern. \end{methoddesc} \begin{memberdesc}[RegexObject]{flags} The flags argument used when the RE object was compiled, or \code{0} if no flags were provided. \end{memberdesc} \begin{memberdesc}[RegexObject]{groupindex} A dictionary mapping any symbolic group names defined by \regexp{(?P<\var{id}>)} to group numbers. The dictionary is empty if no symbolic groups were used in the pattern. \end{memberdesc} \begin{memberdesc}[RegexObject]{pattern} The pattern string from which the RE object was compiled. \end{memberdesc} \subsection{Match Objects \label{match-objects}} \class{MatchObject} instances support the following methods and attributes: \begin{methoddesc}[MatchObject]{expand}{template} Return the string obtained by doing backslash substitution on the template string \var{template}, as done by the \method{sub()} method. Escapes such as \samp{\e n} are converted to the appropriate characters, and numeric backreferences (\samp{\e 1}, \samp{\e 2}) and named backreferences (\samp{\e g<1>}, \samp{\e g}) are replaced by the contents of the corresponding group. \end{methoddesc} \begin{methoddesc}[MatchObject]{group}{\optional{group1, \moreargs}} Returns one or more subgroups of the match. If there is a single argument, the result is a single string; if there are multiple arguments, the result is a tuple with one item per argument. Without arguments, \var{group1} defaults to zero (the whole match is returned). If a \var{groupN} argument 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. If a group number is negative or larger than the number of groups defined in the pattern, an \exception{IndexError} exception is raised. If a group is contained in a part of the pattern that did not match, the corresponding result is \code{None}. If a group is contained in a part of the pattern that matched multiple times, the last match is returned. If the regular expression uses the \regexp{(?P<\var{name}>...)} syntax, the \var{groupN} arguments may also be strings identifying groups by their group name. If a string argument is not used as a group name in the pattern, an \exception{IndexError} exception is raised. A moderately complicated example: \begin{verbatim} m = re.match(r"(?P\d+)\.(\d*)", '3.14') \end{verbatim} After performing this match, \code{m.group(1)} is \code{'3'}, as is \code{m.group('int')}, and \code{m.group(2)} is \code{'14'}. \end{methoddesc} \begin{methoddesc}[MatchObject]{groups}{\optional{default}} Return a tuple containing all the subgroups of the match, from 1 up to however many groups are in the pattern. The \var{default} argument is used for groups that did not participate in the match; it defaults to \code{None}. (Incompatibility note: in the original Python 1.5 release, if the tuple was one element long, a string would be returned instead. In later versions (from 1.5.1 on), a singleton tuple is returned in such cases.) \end{methoddesc} \begin{methoddesc}[MatchObject]{groupdict}{\optional{default}} Return a dictionary containing all the \emph{named} subgroups of the match, keyed by the subgroup name. The \var{default} argument is used for groups that did not participate in the match; it defaults to \code{None}. \end{methoddesc} \begin{methoddesc}[MatchObject]{start}{\optional{group}} \methodline{end}{\optional{group}} Return the indices of the start and end of the substring matched by \var{group}; \var{group} defaults to zero (meaning the whole matched substring). Return \code{-1} if \var{group} exists but did not contribute to the match. For a match object \var{m}, and a group \var{g} that did contribute to the match, the substring matched by group \var{g} (equivalent to \code{\var{m}.group(\var{g})}) is \begin{verbatim} m.string[m.start(g):m.end(g)] \end{verbatim} Note that \code{m.start(\var{group})} will equal \code{m.end(\var{group})} if \var{group} matched a null string. For example, after \code{\var{m} = re.search('b(c?)', 'cba')}, \code{\var{m}.start(0)} is 1, \code{\var{m}.end(0)} is 2, \code{\var{m}.start(1)} and \code{\var{m}.end(1)} are both 2, and \code{\var{m}.start(2)} raises an \exception{IndexError} exception. \end{methoddesc} \begin{methoddesc}[MatchObject]{span}{\optional{group}} For \class{MatchObject} \var{m}, return the 2-tuple \code{(\var{m}.start(\var{group}), \var{m}.end(\var{group}))}. Note that if \var{group} did not contribute to the match, this is \code{(-1, -1)}. Again, \var{group} defaults to zero. \end{methoddesc} \begin{memberdesc}[MatchObject]{pos} The value of \var{pos} which was passed to the \function{search()} or \function{match()} function. This is the index into the string at which the RE engine started looking for a match. \end{memberdesc} \begin{memberdesc}[MatchObject]{endpos} The value of \var{endpos} which was passed to the \function{search()} or \function{match()} function. This is the index into the string beyond which the RE engine will not go. \end{memberdesc} \begin{memberdesc}[MatchObject]{lastgroup} The name of the last matched capturing group, or \code{None} if the group didn't have a name, or if no group was matched at all. \end{memberdesc} \begin{memberdesc}[MatchObject]{lastindex} The integer index of the last matched capturing group, or \code{None} if no group was matched at all. \end{memberdesc} \begin{memberdesc}[MatchObject]{re} The regular expression object whose \method{match()} or \method{search()} method produced this \class{MatchObject} instance. \end{memberdesc} \begin{memberdesc}[MatchObject]{string} The string passed to \function{match()} or \function{search()}. \end{memberdesc} \subsection{Examples} \leftline{\strong{Simulating \cfunction{scanf()}}} Python does not currently have an equivalent to \cfunction{scanf()}. \ttindex{scanf()} Regular expressions are generally more powerful, though also more verbose, than \cfunction{scanf()} format strings. The table below offers some more-or-less equivalent mappings between \cfunction{scanf()} format tokens and regular expressions. \begin{tableii}{l|l}{textrm}{\cfunction{scanf()} Token}{Regular Expression} \lineii{\code{\%c}} {\regexp{.}} \lineii{\code{\%5c}} {\regexp{.\{5\}}} \lineii{\code{\%d}} {\regexp{[-+]?\e d+}} \lineii{\code{\%e}, \code{\%E}, \code{\%f}, \code{\%g}} {\regexp{[-+]?(\e d+(\e.\e d*)?|\e d*\e.\e d+)([eE]\e d+)?}} \lineii{\code{\%i}} {\regexp{[-+]?(0[xX][\e dA-Fa-f]+|0[0-7]*|\e d+)}} \lineii{\code{\%o}} {\regexp{0[0-7]*}} \lineii{\code{\%s}} {\regexp{\e S+}} \lineii{\code{\%u}} {\regexp{\e d+}} \lineii{\code{\%x}, \code{\%X}} {\regexp{0[xX][\e dA-Fa-f]+}} \end{tableii} To extract the filename and numbers from a string like \begin{verbatim} /usr/sbin/sendmail - 0 errors, 4 warnings \end{verbatim} you would use a \cfunction{scanf()} format like \begin{verbatim} %s - %d errors, %d warnings \end{verbatim} The equivalent regular expression would be \begin{verbatim} (\S+) - (\d+) errors, (\d+) warnings \end{verbatim} \leftline{\strong{Avoiding backtracking}} If you create regular expressions that require the engine to perform a lot of backtracking, you may encounter a RuntimeError exception with the message \code{maximum recursion limit exceeded}. For example, \begin{verbatim} >>> s = "<" + "that's a very big string!"*1000 + ">" >>> re.match('<.*?>', s) Traceback (most recent call last): File "", line 1, in ? File "/usr/local/lib/python2.3/sre.py", line 132, in match return _compile(pattern, flags).match(string) RuntimeError: maximum recursion limit exceeded \end{verbatim} You can often restructure your regular expression to avoid backtracking. The above regular expression can be recast as \regexp{\textless[\textasciicircum \textgreater]*\textgreater}. As a further benefit, such regular expressions will run faster than their backtracking equivalents.