Notes on learning Python regular expression grammar

Time:2020-9-19

Regular expression describes a pattern of string matching, which can be used to check whether a string contains a seed string, replace the matched substring, or extract the substring that meets certain conditions from a string.

Python has added the re module since version 1.5, which provides Perl style regular expression patterns.

Re module enables Python language to have all the regular expression functions.

The compile function generates a regular expression object based on a pattern string and optional flag parameters. This object has a series of methods for regular expression matching and replacement.

This article focuses on the python regular expression syntax.

The special characters are:
    “.”      Matches any character except a newline.
    “^”      Matches the start of the string.
    “$”      Matches the end of the string or just before the newline at
             the end of the string.
    “*”      Matches 0 or more (greedy) repetitions of the preceding RE.
             Greedy means that it will match as many repetitions as possible.
    “+”      Matches 1 or more (greedy) repetitions of the preceding RE.
    “?”      Matches 0 or 1 (greedy) of the preceding RE.
    *?,+?,?? Non-greedy versions of the previous three special characters.
    {m,n}    Matches from m to n repetitions of the preceding RE.
    {m,n}?   Non-greedy version of the above.
    “\\”     Either escapes special characters or signals a special sequence.
    []       Indicates a set of characters.
             A “^” as the first character indicates a complementing set.
    “|”      A|B, creates an RE that will match either A or B.
    (…)    Matches the RE inside the parentheses.
             The contents can be retrieved or matched later in the string.
    (?aiLmsux) Set the A, I, L, M, S, U, or X flag for the RE (see below).
    (?:…)  Non-grouping version of regular parentheses.
    (?P<name>…) The substring matched by the group is accessible by name.
    (?P=name)     Matches the text matched earlier by the group named name.
    (?#…)  A comment; ignored.
    (?=…)  Matches if … matches next, but doesn’t consume the string.
    (?!…)  Matches if … doesn’t match next.
    (?<=…) Matches if preceded by … (must be fixed length).
    (?<!…) Matches if not preceded by … (must be fixed length).
    (?(id/name)yes|no) Matches yes pattern if the group with id/name matched,
                       the (optional) no pattern otherwise.

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.
    \number  Matches the contents of the group of the same number.
    \A       Matches only at the start of the string.
    \Z       Matches only at the end of the string.
    \b       Matches the empty string, but only at the start or end of a word.
    \B       Matches the empty string, but not at the start or end of a word.
    \d       Matches any decimal digit; equivalent to the set [0-9] in
             bytes patterns or string patterns with the ASCII flag.
             In string patterns without the ASCII flag, it will match the whole
             range of Unicode digits.
    \D       Matches any non-digit character; equivalent to [^\d].
    \s       Matches any whitespace character; equivalent to [ \t\n\r\f\v] in
             bytes patterns or string patterns with the ASCII flag.
             In string patterns without the ASCII flag, it will match the whole
             range of Unicode whitespace characters.
    \S       Matches any non-whitespace character; equivalent to [^\s].
    \w       Matches any alphanumeric character; equivalent to [a-zA-Z0-9_]
             in bytes patterns or string patterns with the ASCII flag.
             In string patterns without the ASCII flag, it will match the
             range of Unicode alphanumeric characters (letters plus digits
             plus underscore).
             With LOCALE, it will match the set [0-9_] plus characters defined
             as letters for the current locale.
    \W       Matches the complement of \w.
    \\       Matches a literal backslash.

This module exports the following functions:
    match     Match a regular expression pattern to the beginning of a string.
    fullmatch Match a regular expression pattern to all of a string.
    search    Search a string for the presence of a pattern.
    sub       Substitute occurrences of a pattern found in a string.
    subn      Same as sub, but also return the number of substitutions made.
    split     Split a string by the occurrences of a pattern.
    findall   Find all occurrences of a pattern in a string.
    finditer  Return an iterator yielding a match object for each match.
    compile   Compile a pattern into a RegexObject.
    purge     Clear the regular expression cache.
    escape    Backslash all non-alphanumerics in a string.

Some of the functions in this module takes flags as optional parameters:
    A  ASCII       For string patterns, make \w, \W, \b, \B, \d, \D
                   match the corresponding ASCII character categories
                   (rather than the whole Unicode categories, which is the
                   default).
                   For bytes patterns, this flag is the only available
                   behaviour and needn’t be specified.
    I  IGNORECASE  Perform case-insensitive matching.
    L  LOCALE      Make \w, \W, \b, \B, dependent on the current locale.
    M  MULTILINE   “^” matches the beginning of lines (after a newline)
                   as well as the string.
                   “$” matches the end of lines (before a newline) as well
                   as the end of the string.
    S  DOTALL      “.” matches any character at all, including the newline.
    X  VERBOSE     Ignore whitespace and comments for nicer looking RE’s.
    U  UNICODE     For compatibility only. Ignored for string patterns (it
                   is the default), and forbidden for bytes patterns.

 

Let’s take a look at the regular expression matching process:

The general matching process of regular expression is: take out the characters in the expression and the text in turn. If each character can match, the matching will succeed; if there are unsuccessful characters, the matching will fail. If there are quantifiers or boundaries in the expression, the process will be slightly different, but it is also easy to understand. You can understand it by using it several times.

summary

This article about Python regular expression syntax record of this article introduced here, more related Python regular expression syntax record content, please search the previous articles of developeppaer or continue to browse the related articles below, I hope you can support developeppaer more in the future!