Regular Expressions 101

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An explanation of your regex will be automatically generated as you type.
Detailed match information will be displayed here automatically.
  • All Tokens
  • Common Tokens
  • General Tokens
  • Anchors
  • Meta Sequences
  • Quantifiers
  • Group Constructs
  • Character Classes
  • Flags/Modifiers
  • Substitution
  • A single character of: a, b or c
    [abc]
  • A character except: a, b or c
    [^abc]
  • A character in the range: a-z
    [a-z]
  • A character not in the range: a-z
    [^a-z]
  • A character in the range: a-z or A-Z
    [a-zA-Z]
  • Any single character
    .
  • Alternate - match either a or b
    a|b
  • Any whitespace character
    \s
  • Any non-whitespace character
    \S
  • Any digit
    \d
  • Any non-digit
    \D
  • Any word character
    \w
  • Any non-word character
    \W
  • Non-capturing group
    (?:...)
  • Capturing group
    (...)
  • Zero or one of a
    a?
  • Zero or more of a
    a*
  • One or more of a
    a+
  • Exactly 3 of a
    a{3}
  • 3 or more of a
    a{3,}
  • Between 3 and 6 of a
    a{3,6}
  • Start of string
    ^
  • End of string
    $
  • A word boundary
    \b
  • Non-word boundary
    \B

Regular Expression
No Match

/
/
gm

Test String

Code Generator

Language

Generated Code

# coding=utf8 # the above tag defines encoding for this document and is for Python 2.x compatibility import re regex = r"^([A-z]+)(?: *= *)(\d+|[A-z]+)$" test_str = ("Expression validator\n" "^((?:[A-z]+)|((-|\\+)?\\d+))( +(\\++|-+) +((?:-?\\d+)|(?:[A-z]+)))*$\n\n" "Assignment validator\n" "^([A-z]+)(?: *= *)(\\d+|[A-z]+)$\n\n" "c= a\n" "8\n" "8 + 4\n" "-2 + 4 - 5 + 6\n\n" "9 +++ - 10 -- 8\n" "9 --- + 10\n" "9 --- -10\n" "9 +++ -10 -- 8\n" "3 --- 5\n" "14 - 12\n" "-3 2\n" "name = 44\n" "a = 5\n" "capName = 6 + 4\n" "capName = 6\n" "n = 3\n" "m=4\n" "a = 5\n" "b = a\n" "v= 7\n" "n =9\n" "c= a\n" "A variable can have a name consisting of more than one letter.\n\n" "count = 10\n" "To print the value of a variable you should just type its name.\n\n" "N = 5\n" "N\n" "5\n" "It should be possible to set a new value to an existing variable.\n\n" "a = 1\n" "a = 2\n" "a = 3\n" "a\n" "3\n" "If an identifier or value of a variable is invalid, the program must print a message like the one below.\n\n" "a1 = 8\n" "Invalid identifier\n" "n = a2a\n" "Invalid assignment\n" "a = 7 = 8\n" "Invalid assignment\n\n\n") matches = re.finditer(regex, test_str, re.MULTILINE) for matchNum, match in enumerate(matches, start=1): print ("Match {matchNum} was found at {start}-{end}: {match}".format(matchNum = matchNum, start = match.start(), end = match.end(), match = match.group())) for groupNum in range(0, len(match.groups())): groupNum = groupNum + 1 print ("Group {groupNum} found at {start}-{end}: {group}".format(groupNum = groupNum, start = match.start(groupNum), end = match.end(groupNum), group = match.group(groupNum))) # Note: for Python 2.7 compatibility, use ur"" to prefix the regex and u"" to prefix the test string and substitution.

Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Python, please visit: https://docs.python.org/3/library/re.html