\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 tothose found in Perl. Regular expression pattern strings may notcontain null bytes, but can specify the null byte using the\code{\e\var{number}} notation. Both patterns and strings to besearched 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}) toindicate special forms or to allow special characters to be usedwithout invoking their special meaning. This collides with Python'susage 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 expressionmust 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 regularexpression patterns; backslashes are not handled in any special way ina string literal prefixed with \character{r}. So \code{r"\e n"} is atwo-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 rawstring 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 matchesit; the functions in this module let you check if a particular stringmatches a given regular expression (or if a given regular expressionmatches a particular string, which comes down to the same thing). Regular expressions can be concatenated to form new regularexpressions; 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, complexexpressions can easily be constructed from simpler primitiveexpressions like the ones described here. For details of the theoryand implementation of regular expressions, consult the Friedl bookreferenced above, or almost any textbook about compiler construction. A brief explanation of the format of regular expressions follows. Forfurther information and a gentler presentation, consult the RegularExpression 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 matchthemselves. You can concatenate ordinary characters, so \regexp{last}matches the string \code{'last'}. (In the rest of this section, we'llwrite RE's in \regexp{this special style}, usually without quotes, andstrings to be matched \code{'in single quotes'}.) Some characters, like \character{|} or \character{(}, are special.Special characters either stand for classes of ordinary characters, oraffect how the regular expressions around them are interpreted. The special characters are:%\begin{description} \item[\character{.}] (Dot.) In the default mode, this matches anycharacter except a newline. If the \constant{DOTALL} flag has beenspecified, this matches any character including a newline. \item[\character{\textasciicircum}] (Caret.) Matches the start of thestring, and in \constant{MULTILINE} mode also matches immediatelyafter each newline. \item[\character{\$}] Matches the end of the string or just before thenewline at the end of the string, and in \constant{MULTILINE} modealso 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 tomatch 0 or more repetitions of the preceding RE, as many repetitionsas are possible. \regexp{ab*} willmatch 'a', 'ab', or 'a' followed by any number of 'b's. \item[\character{+}] Causes theresulting 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; itwill not match just 'a'. \item[\character{?}] Causes the resulting RE tomatch 0 or 1 repetitions of the preceding RE. \regexp{ab?} willmatch either 'a' or 'ab'. \item[\code{*?}, \code{+?}, \code{??}] The \character{*},\character{+}, and \character{?} qualifiers are all \dfn{greedy}; theymatch as much text as possible. Sometimes this behaviour isn'tdesired; if the RE \regexp{<.*>} is matched against\code{'<H1>title</H1>'}, it will match the entire string, and not just\code{'<H1>'}. Adding \character{?} after the qualifier makes itperform 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{'<H1>'}. \item[\code{\{\var{m}\}}]Specifies that exactly \var{m} copies of the previous RE should bematched; fewer matches cause the entire RE not to match. For example,\regexp{a\{6\}} will match exactly six \character{a} characters, butnot 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 tomatch 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 anexample, \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 withthe previously described form. \item[\code{\{\var{m},\var{n}\}?}] Causes the resulting RE tomatch from \var{m} to \var{n} repetitions of the preceding RE,attempting to match as \emph{few} repetitions as possible. This isthe non-greedy version of the previous qualifier. For example, on the6-character string \code{'aaaaaa'}, \regexp{a\{3,5\}} will match 5\character{a} characters, while \regexp{a\{3,5\}?} will only match 3characters. \item[\character{\e}] Either escapes special characters (permittingyou to match characters like \character{*}, \character{?}, and soforth), or signals a special sequence; special sequences are discussedbelow. If you're not using a raw string toexpress the pattern, remember that Python also uses thebackslash as an escape sequence in string literals; if the escapesequence isn't recognized by Python's parser, the backslash andsubsequent character are included in the resulting string. However,if Python would recognize the resulting sequence, the backslash shouldbe repeated twice. This is complicated and hard to understand, soit's highly recommended that you use raw strings for all but thesimplest expressions. \item[\code{[]}] Used to indicate a set of characters. Characters canbe listed individually, or a range of characters can be indicated bygiving two characters and separating them by a \character{-}. Specialcharacters 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 anyletter or digit. Character classes such as \code{\e w} or \code{\e S}(defined below) are also acceptable inside a range. If you want toinclude a \character{]} or a \character{-} inside a set, precede it with abackslash, or place it as the first character. Thepattern \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 matchany character except \character{5}, and\regexp{[\textasciicircum\code{\textasciicircum}]} will match any characterexcept \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. Anarbitrary number of REs can be separated by the \character{|} in thisway. This can be used inside groups (see below) as well. As the targetstring is scanned, REs separated by \character{|} are tried from left toright. 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 theparentheses, and indicates the start and end of a group; the contentsof a group can be retrieved after a match has been performed, and canbe matched later in the string with the \regexp{\e \var{number}} specialsequence, described below. To match the literals \character{(} or\character{)}, use \regexp{\e(} or \regexp{\e)}, or enclose theminside a character class: \regexp{[(] [)]}. \item[\code{(?...)}] This is an extension notation (a \character{?}following a \character{(} is not meaningful otherwise). The firstcharacter 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 setthe 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 toinclude the flags as part of the regular expression, instead ofpassing 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 morewhitespace characters. If there are non-whitespace characters beforethe flag, the results are undefined. \item[\code{(?:...)}] A non-grouping version of regular parentheses.Matches whatever regular expression is inside the parentheses, but thesubstring matched by thegroup \emph{cannot} be retrieved after performing a match orreferenced later in the pattern. \item[\code{(?P<\var{name}>...)}] Similar to regular parentheses, butthe substring matched by the group is accessible via the symbolic groupname \var{name}. Group names must be valid Python identifiers, andeach group name must be defined only once within a regular expression. Asymbolic group is also a numbered group, just as if the group were notnamed. So the group named 'id' in the example above can also bereferenced as the numbered group 1. For example, if the pattern is\regexp{(?P<id>[a-zA-Z_]\e w*)}, the group can be referenced by itsname in arguments to methods of match objects, such as\code{m.group('id')} or \code{m.end('id')}, and also by name inpattern text (for example, \regexp{(?P=id)}) and replacement text(such as \code{\e g<id>}). \item[\code{(?P=\var{name})}] Matches whatever text was matched by theearlier group named \var{name}. \item[\code{(?\#...)}] A comment; the contents of the parentheses aresimply ignored. \item[\code{(?=...)}] Matches if \regexp{...} matches next, but doesn'tconsume any of the string. This is called a lookahead assertion. Forexample, \regexp{Isaac (?=Asimov)} will match \code{'Isaac~'} only if it'sfollowed by \code{'Asimov'}. \item[\code{(?!...)}] Matches if \regexp{...} doesn't match next. Thisis 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 stringis preceded by a match for \regexp{...} that ends at the currentposition. This is called a \dfn{positive lookbehind assertion}.\regexp{(?<=abc)def} will find a match in \samp{abcdef}, since thelookbehind will back up 3 characters and check if the containedpattern matches. The contained pattern must only match strings ofsome fixed length, meaning that \regexp{abc} or \regexp{a|b} areallowed, but \regexp{a*} and \regexp{a\{3,4\}} are not. Note thatpatterns which start with positive lookbehind assertions will nevermatch at the beginning of the string being searched; you will mostlikely 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{(?<!...)}] Matches if the current position in the stringis not preceded by a match for \regexp{...}. This is called a\dfn{negative lookbehind assertion}. Similar to positive lookbehindassertions, the contained pattern must only match strings of somefixed length. Patterns which start with negative lookbehindassertions may match at the beginning of the string being searched. \item[\code{(?(\var{id/name})yes-pattern|no-pattern)}] Will try to matchwith \regexp{yes-pattern} if the group with given \var{id} or \var{name}exists, and with \regexp{no-pattern} if it doesn't. \regexp{|no-pattern}is optional and can be omitted. For example, \regexp{(<)?(\e w+@\e w+(?:\e .\e w+)+)(?(1)>)} is a poor email matchingpattern, which will match with \code{'<user@host.com>'} as well as\code{'user@host.com'}, but not with \code{'<user@host.com'}.\versionadded{2.4} \end{description} The special sequences consist of \character{\e} and a character from thelist below. If the ordinary character is not on the list, then theresulting RE will match the second character. For example,\regexp{\e\$} matches the character \character{\$}.%\begin{description} \item[\code{\e \var{number}}] Matches the contents of the group of thesame number. Groups are numbered starting from 1. For example,\regexp{(.+) \e 1} matches \code{'the the'} or \code{'55 55'}, but not\code{'the end'} (notethe space after the group). This special sequence can only be used tomatch 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 be interpretedas a group match, but as the character with octal value \var{number}.Inside the \character{[} and \character{]} of a character class, all numericescapes 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 thebeginning or end of a word. A word is defined as a sequence ofalphanumeric or underscore characters, so the end of a word is indicated bywhitespace or a non-alphanumeric, non-underscore character. Note that {}\code{\e b} is defined as the boundary between \code{\e w} and \code{\eW}, so the precise set of characters deemed to be alphanumeric depends on thevalues of the \code{UNICODE} and \code{LOCALE} flags. Inside a characterrange, \regexp{\e b} represents the backspace character, for compatibilitywith 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{\eb}, so is also subject to the settings of \code{LOCALE} and \code{UNICODE}. \item[\code{\e d}]When the \constant{UNICODE} flag is not specified, matchesany decimal digit; this is equivalent to the set \regexp{[0-9]}. With \constant{UNICODE}, it will match whatever is classified as a digitin the Unicode character properties database. \item[\code{\e D}]When the \constant{UNICODE} flag is not specified, matchesany non-digit character; this is equivalent to the set \regexp{[{\textasciicircum}0-9]}. With \constant{UNICODE}, it will match anything other than character marked as digits in the Unicode character properties database. \item[\code{\e s}]When the \constant{LOCALE} and \constant{UNICODE}flags are not specified, matches any whitespace character; this isequivalent to the set \regexp{[ \e t\e n\e r\e f\e v]}.With \constant{LOCALE}, it will match this set plus whatever charactersare defined as space for the current locale. If \constant{UNICODE} is set,this will match the characters \regexp{[ \e t\e n\e r\e f\e v]} pluswhatever is classified as space in the Unicode character propertiesdatabase. \item[\code{\e S}]When the \constant{LOCALE} and \constant{UNICODE}flags are not specified, matches any non-whitespace character; this isequivalent to the set \regexp{[\textasciicircum\ \e t\e n\e r\e f\e v]}With \constant{LOCALE}, it will match any character not in this set,and not defined as space in the current locale. If \constant{UNICODE}is set, this will match anything other than \regexp{[ \e t\e n\e r\e f\e v]}and characters marked as space in the Unicode character properties database. \item[\code{\e w}]When the \constant{LOCALE} and \constant{UNICODE}flags are not specified, matches any alphanumeric character and theunderscore; 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 forthe current locale. If \constant{UNICODE} is set, this will match thecharacters \regexp{[0-9_]} plus whatever is classified as alphanumericin 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; thisis 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 Unicodecharacter properties database. \item[\code{\e Z}]Matches only at the end of the string. \end{description} Most of the standard escapes supported by Python string literals arealso 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 a0, or if there are three octal digits, it is considered an octalescape. Otherwise, it is a group reference. As for string literals,octal escapes are always at most three digits in length. % 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 regularexpressions: match and search. If you are accustomed to Perl'ssemantics, the search operation is what you're looking for. See the\function{search()} function and corresponding method of compiledregular expression objects. Note that match may differ from search using a regular expressionbeginning with \character{\textasciicircum}:\character{\textasciicircum} matches only at thestart of the string, or in \constant{MULTILINE} mode also immediatelyfollowing a newline. The ``match'' operation succeeds only if thepattern matches at the start of the string regardless of mode, or atthe starting position given by the optional \var{pos} argumentregardless of whether a newline precedes it. % Examples from Tim Peters:\begin{verbatim}re.compile("a").match("ba", 1) # succeedsre.compile("^a").search("ba", 1) # fails; 'a' not at startre.compile("^a").search("\na", 1) # fails; 'a' not at startre.compile("^a", re.M).search("\na", 1) # succeedsre.compile("^a", re.M).search("ba", 1) # fails; no preceding \n\end{verbatim} \subsection{Module Contents}\nodename{Contents of Module re} The module defines several functions, constants, and an exception. Some of thefunctions are simplified versions of the full featured methods for compiledregular expressions. Most non-trivial applications always use the compiledform. \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 theexpression 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 thecurrent locale.\end{datadesc} \begin{datadesc}{L}\dataline{LOCALE}Make \regexp{\e w}, \regexp{\e W}, \regexp{\e b}, \regexp{\e B},\regexp{\e s} and \regexp{\e S} 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 eachline (immediately following each newline); and the pattern character\character{\$} matches at the end of the string and at the end of eachline (immediately preceding each newline). By default,\character{\textasciicircum} matches only at the beginning of thestring, and \character{\$} only at the end of the string andimmediately before the newline (if any) at the end of the string.Lines 1–500 of 954. Load more
\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 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{description} \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{'<H1>title</H1>'}, it will match the entire string, and not just \code{'<H1>'}. 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{'<H1>'}. \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<id>[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<id>}). \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{(?<!...)}] Matches if the current position in the string is not preceded by a match for \regexp{...}. This is called a \dfn{negative lookbehind assertion}. Similar to positive lookbehind assertions, the contained pattern must only match strings of some fixed length. Patterns which start with negative lookbehind assertions may match at the beginning of the string being searched. \item[\code{(?(\var{id/name})yes-pattern|no-pattern)}] Will try to match with \regexp{yes-pattern} if the group with given \var{id} or \var{name} exists, and with \regexp{no-pattern} if it doesn't. \regexp{|no-pattern} is optional and can be omitted. For example, \regexp{(<)?(\e w+@\e w+(?:\e .\e w+)+)(?(1)>)} is a poor email matching pattern, which will match with \code{'<user@host.com>'} as well as \code{'user@host.com'}, but not with \code{'<user@host.com'}. \versionadded{2.4} \end{description} The special sequences consist of \character{\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, \regexp{\e\$} matches the character \character{\$}. % \begin{description} \item[\code{\e \var{number}}] Matches the contents of the group of the same number. Groups are numbered starting from 1. For example, \regexp{(.+) \e 1} matches \code{'the the'} or \code{'55 55'}, but not \code{'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 be interpreted as a group match, but as the character with octal value \var{number}. 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}]When the \constant{UNICODE} flag is not specified, matches any decimal digit; this is equivalent to the set \regexp{[0-9]}. With \constant{UNICODE}, it will match whatever is classified as a digit in the Unicode character properties database. \item[\code{\e D}]When the \constant{UNICODE} flag is not specified, matches any non-digit character; this is equivalent to the set \regexp{[{\textasciicircum}0-9]}. With \constant{UNICODE}, it will match anything other than character marked as digits in the Unicode character properties database. \item[\code{\e s}]When the \constant{LOCALE} and \constant{UNICODE} flags are not specified, matches any whitespace character; this is equivalent to the set \regexp{[ \e t\e n\e r\e f\e v]}. With \constant{LOCALE}, it will match this set plus whatever characters are defined as space for the current locale. If \constant{UNICODE} is set, this will match the characters \regexp{[ \e t\e n\e r\e f\e v]} plus whatever is classified as space in the Unicode character properties database. \item[\code{\e S}]When the \constant{LOCALE} and \constant{UNICODE} flags are not specified, matches any non-whitespace character; this is equivalent to the set \regexp{[\textasciicircum\ \e t\e n\e r\e f\e v]} With \constant{LOCALE}, it will match any character not in this set, and not defined as space in the current locale. If \constant{UNICODE} is set, this will match anything other than \regexp{[ \e t\e n\e r\e f\e v]} and characters marked as space in the Unicode character properties database. \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{description} 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. As for string literals, octal escapes are always at most three digits in length. % 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 several functions, constants, and an exception. Some of the functions are simplified versions of the full featured methods for compiled regular expressions. Most non-trivial applications always use the compiled form. \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}, \regexp{\e B}, \regexp{\e s} and \regexp{\e S} 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.