Regular Expressions 101

Save & Share

Flavor

  • PCRE2 (PHP >=7.3)
  • PCRE (PHP <7.3)
  • ECMAScript (JavaScript)
  • Python
  • Golang
  • Java 8

Function

  • Match
  • Substitution
  • List
  • Unit Tests
"
(\d)(?=(\d{3})+$)
"
gm
1st Capturing Group
(\d)
\d matches a digit (equivalent to [0-9])
Positive Lookahead
(?=(\d{3})+$)
Assert that the Regex below matches
2nd Capturing Group
(\d{3})+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
\d
matches a digit (equivalent to [0-9])
{3} matches the previous token exactly 3 times
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"
gm

Test String

Substitution

Processing...