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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 second edition of the book no longer covers Python at all, but the first edition covered 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. In general, if a string \emph{p} matches \emph{A} and another string \emph{q} matches \emph{B}, the string \emph{pq} will match AB. This holds unless \emph{A} or \emph{B} contain low precedence operations; boundary conditions between \emph{A} and \emph{B}; or have numbered group references. 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 above, 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{m} specifies a lower bound of zero, and omitting \var{n} specifies an infinite upper bound. As an example, \regexp{a\{4,\}b} will match \code{aaaab} or 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. As the target string is scanned, REs separated by \character{|} are tried from left to right. When one pattern completely matches, that branch is accepted. This means that once \code{A} matches, \code{B} will not be tested further, 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{(?)} is a poor email matching pattern, which will match with \code{''} as well as \code{'user@host.com'}, but not with \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 unless they touch the beginning of another match. \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 unless they touch the beginning of another match. \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]{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]{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]{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 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()} method of the \class{RegexObject}. 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()} method of the \class{RegexObject}. This is the index into the string beyond which the RE engine will not go. \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. For example, the expressions \regexp{(a)b}, \regexp{((a)(b))}, and \regexp{((ab))} will have \code{lastindex == 1} if applyied to the string \code{'ab'}, while the expression \regexp{(a)(b)} will have \code{lastindex == 2}, if applyied to the same string. \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]{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 recursion}} If you create regular expressions that require the engine to perform a lot of recursion, you may encounter a RuntimeError exception with the message \code{maximum recursion limit} exceeded. For example, \begin{verbatim} >>> import re >>> s = 'Begin ' + 1000*'a very long string ' + 'end' >>> re.match('Begin (\w| )*? end', s).end() 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 recursion. Starting with Python 2.3, simple uses of the \regexp{*?} pattern are special-cased to avoid recursion. Thus, the above regular expression can avoid recursion by being recast as \regexp{Begin [a-zA-Z0-9_ ]*?end}. As a further benefit, such regular expressions will run faster than their recursive equivalents.