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To test a regular expression, first search for errors such as non-escaped characters or unbalanced parentheses. Then test it against various input strings to ensure it accepts correct strings and regex wrong ones. A regex tester tool is a great tool that does all of this.
test() The test() method executes a search for a match between a regular expression and a specified string.
The three basic operations in which regular expressions are used are: matching (Does this (entire) string match this pattern?) searching (Is this pattern found within this string?) transforming (such as replacing one or all occurrences of a pattern with another string)
match() function of re in Python will search the regular expression pattern and return the first occurrence. The Python RegEx Match method checks for a match only at the beginning of the string. So, if a match is found in the first line, it returns the match object.
The most powerful free online regexp testing tool is by far http://regex101.com/ - lets you select the RE engine (PCRE, JavaScript, Python), has a debugger, colorizes the matches, explains the regexp on the fly, can create permalinks to the regex playground.
Other online tools:
Windows desktop tools:
Jeff Atwood [wrote about regular expressions]( post:).
Other tools recommended by SO users include:
RegexBuddy
I use Expresso (www.ultrapico.com). It has a lot of nice features for the developer. The Regulator used to be my favorite, but it hasn't been updated in so long and I constantly ran into crashes with complicated RegExs.
Here are some for the Mac: (Note: don't judge the tools by their websites)
If you are an Emacs user, the command re-builder lets you type an Emacs regex and shows on the fly the matching strings in the current buffer, with colors to mark groups. It's free as Emacs.
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