## 4.2.1 Regular Expression 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 A and B are both regular expressions, then AB is also an regular expression. If a string p matches A and another string q matches B, the string 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.

Regular expressions can contain both special and ordinary characters. Most ordinary characters, like '`A`', '`a`', or '`0`', are the simplest regular expressions; they simply match themselves. You can concatenate ordinary characters, so '`last`' matches the characters 'last'. (In the rest of this section, we'll write RE's in `this special font`, usually without quotes, and strings to be matched 'in single quotes'.)

Some characters, like `|` or `(`, 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:

`.`
  (Dot.) In the default mode, this matches any character except a newline. If the `DOTALL` flag has been specified, this matches any character including a newline.
`^`
  (Caret.) Matches the start of the string, and in `MULTILINE` mode also immediately after each newline.
`$`
  Matches the end of the string, and in `MULTILINE` mode also matches before a newline. `foo` matches both 'foo' and 'foobar', while the regular expression `foo$` matches only 'foo'.
`*`
  Causes the resulting RE to match 0 or more repetitions of the preceding RE, as many repetitions as are possible. `ab*` will match 'a', 'ab', or 'a' followed by any number of 'b's.
`+`
  Causes the resulting RE to match 1 or more repetitions of the preceding RE. `ab+` will match 'a' followed by any non-zero number of 'b's; it will not match just 'a'.
`?`
  Causes the resulting RE to match 0 or 1 repetitions of the preceding RE. `ab?` will match either 'a' or 'ab'.
`*?`, `+?`, `??`
  The `*`, `+`, and `?` qualifiers are all greedy; they match as much text as possible. Sometimes this behaviour isn't desired; if the RE `<.*>` is matched against `<H1>title</H1>`, it will match the entire string, and not just `<H1>`. Adding `?` after the qualifier makes it perform the match in non-greedy or minimal fashion; as few characters as possible will be matched. Using `.*?` in the previous expression will match only `<H1>`.
`{m,n}`
  Causes the resulting RE to match from m to n repetitions of the preceding RE, attempting to match as many repetitions as possible. For example, `a{3,5}` will match from 3 to 5 'a' characters.
`{m,n}?`
  Causes the resulting RE to match from m to n repetitions of the preceding RE, attempting to match as few repetitions as possible. This is the non-greedy version of the previous qualifier. For example, on the 6-character string 'aaaaaa', `a{3,5}` will match 5 'a' characters, while `a{3,5}?` will only match 3 characters.
`\`
  Either escapes special characters (permitting you to match characters like '*?+&$'), or signals a special sequence; special sequences are discussed below.

  If you're not using a raw string to express the pattern, remember that Python also uses the backslash as an escape sequence in string literals; if the escape sequence isn't recognized by Python's parser, the backslash and subsequent character are included in the resulting string. However, if Python would recognize the resulting sequence, the backslash should be repeated twice. This is complicated and hard to understand, so it's highly recommended that you use raw strings for all but the simplest expressions.

`[]`
  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 '-'. Special characters are not active inside sets. For example, `[akm$]` will match any of the characters 'a', 'k', 'm', or '$'; `[a-z]` will match any lowercase letter and `[a-zA-Z0-9]` matches any letter or digit. Character classes such as `\w` or `\ S` (defined below) are also acceptable inside a range. If you want to include a `]` or a `-` inside a set, precede it with a backslash.

  Characters not within a range can be matched by including a `^` as the first character of the set; `^` elsewhere will simply match the '`^`' character.

`|`
  `A|B`, where A and B can be arbitrary REs, creates a regular expression that will match either A or B. This can be used inside groups (see below) as well. To match a literal '`|`', use `\|`, or enclose it inside a character class, like `[|]`.
`(...)`
  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 `\number` special sequence, described below. To match the literals '(' or ')', use `\(` or `\)`, or enclose them inside a character class: `[(] [)]`.
`(?...)`
  This is an extension notation (a '?' following a '(' is not meaningful otherwise). The first character after the '?' determines what the meaning and further syntax of the construct is. Following are the currently supported extensions.
`(?iLmsx)`
  (One or more letters from the set 'i', 'L', 'm', 's', 'x'.) The group matches the empty string; the letters set the corresponding flags (re.I, re.L, re.M, re.S, re.X) for the entire regular expression. This is useful if you wish include the flags as part of the regular expression, instead of passing a flag argument to the `compile` function.
`(?:...)`
  A non-grouping version of regular parentheses. Matches whatever's inside the parentheses, but the text matched by the group cannot be retrieved after performing a match or referenced later in the pattern.
`(?P<name>...)`
  Similar to regular parentheses, but the text matched by the group is accessible via the symbolic group name 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 `(?P<id>[a-zA-Z_]\w*)`, the group can be referenced by its name in arguments to methods of match objects, such as `m.group('id')` or `m.end('id')`, and also by name in pattern text (e.g. `(?P=id)`) and replacement text (e.g. `\g<id>`).

`(?P=name)`
  Matches whatever text was matched by the earlier group named name.
`(?#...)`
  A comment; the contents of the parentheses are simply ignored.
`(?=...)`
  Matches if `...` matches next, but doesn't consume any of the string. This is called a lookahead assertion. For example, `Isaac (?=Asimov)` will match 'Isaac ' only if it's followed by 'Asimov'.
`(?!...)`
  Matches if `...` doesn't match next. This is a negative lookahead assertion. For example, For example, `Isaac (?!Asimov)` will match 'Isaac ' only if it's not followed by 'Asimov'.

The special sequences consist of '`\`' 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, `\$` matches the character '$'.

`\number`
  Matches the contents of the group of the same number. Groups are numbered starting from 1. For example, `(.+) \1` matches 'the the' or '55 55', but not 'the end' (note the space after the group). This special sequence can only be used to match one of the first 99 groups. If the first digit of number is 0, or number is 3 octal digits long, it will not be interpreted as a group match, but as the character with octal value number.
`\A`
  Matches only at the start of the string.
`\b`
  Matches the empty string, but only at the beginning or end of a word. A word is defined as a sequence of alphanumeric characters, so the end of a word is indicated by whitespace or a non-alphanumeric character. Inside a character range, `\b` represents the backspace character, for compatibility with Python's string literals.
`\B`
  Matches the empty string, but only when it is not at the beginning or end of a word.
`\d`
  Matches any decimal digit; this is equivalent to the set `[0-9]`.
`\D`
  Matches any non-digit character; this is equivalent to the set `[00-9]`.
`\s`
  Matches any whitespace character; this is equivalent to the set `[ \t\n\r\f\v]`.
`\S`
  Matches any non-whitespace character; this is equivalent to the set `[^\t\n\r\f\v]`.
`\w`
  When the LOCALE flag is not specified, matches any alphanumeric character; this is equivalent to the set `[a-zA-Z0-9_]`. With LOCALE, it will match the set `[0-9_]` plus whatever characters are defined as letters for the current locale.
`\W`
  When the LOCALE flag is not specified, matches any non-alphanumeric character; this is equivalent to the set `[â-zA-Z0-9_]`. With LOCALE, it will match any character not in the set `[0-9_]`, and not defined as a letter for the current locale.

`\Z`
  Matches only at the end of the string.

`\\`
  Matches a literal backslash.