\section{\module{re} --- Regular expression operations} \declaremodule{standard}{re} \moduleauthor{Andrew M. Kuchling}{amk1@bigfoot.com} \moduleauthor{Fredrik Lundh}{effbot@telia.com} \sectionauthor{Andrew M. Kuchling}{amk1@bigfoot.com} \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. \strong{Implementation note:} The \module{re}\refstmodindex{pre} module has two distinct implementations: \module{sre} is the default implementation and includes Unicode support, but may run into stack limitations for some patterns. Though this will be fixed for a future release of Python, the older implementation (without Unicode support) is still available as the \module{pre}\refstmodindex{pre} module. \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 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 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{\^}] (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, 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'. \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},\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}. \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{\^} as the first character of the set; \character{\^} elsewhere will simply match the \character{\^} character. For example, \regexp{[{\^}5]} will match any character except \character{5}. \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. 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 positive lookbehind assertion. \regexp{(?<=abc)def} will match \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*} isn't. \item[\code{(?>> 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}{sub}{pattern, repl, string\optional{, count\code{ = 0}}} 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 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, 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 \samp{sub('x*', '-', 'abc')} returns \code{'-a-b-c-'}. If \var{repl} 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{\e 6}, are replaced with the substring matched by group 6 in the pattern. 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}. \end{funcdesc} \begin{funcdesc}{subn}{pattern, repl, string\optional{, count\code{ = 0}}} 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. \strong{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{'\^'} 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 \var{endpos} will be searched for a match. \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]{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}} \funcline{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}