A regular expression is faster to write than to trust. It looks right, matches your three examples, and then quietly fails on the fourth in production. Regex Lab closes that loop: matches highlight in the test string as you type, every capture group is broken out per match, and a broken pattern shows its error instead of throwing. Nothing you paste leaves the page.
Build it against real text
Start from a job. Say you are pulling the parts out of an ISO date. Paste a few lines of real input into the test area — including the ugly ones — and write the pattern with parentheses around the pieces you want:
(\d{4})-(\d{2})-(\d{2})
The whole match highlights, and Regex Lab lists group 1, 2 and 3 for each line: the year, month and day. If a line does not match, it simply does not highlight, which is how you find the input your pattern forgot about.
Name the groups so the code reads
Numbered groups are fine until you have five of them and cannot remember which is which. Named groups fix that, and they carry straight into most languages:
(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})
Now the group panel shows year, month and day by name. Same match, far more readable code downstream.
Preview the replace before you run it
The replacement pane is where Regex Lab earns its keep. Type a replacement using back-references and watch the substituted text appear, so you never run a destructive find-and-replace blind. To flip that ISO date to day/month/year:
$3/$2/$1
The result updates live. With named groups you can write the dollar-and-angle-bracket form instead, and the preview shows exactly what your editor's replace would produce.
The traps that bite
Greedy versus lazy. By default quantifiers are greedy — they grab as much as possible. Matching a quoted string with a dot-star between quotes will, on a line with two quoted words, swallow everything from the first quote to the last. Make the quantifier lazy by adding a question mark after it so it stops at the first closing quote. Watching the highlight stretch across too much text is how you catch this instantly.
Anchors. A pattern with no anchors matches anywhere in the line, so a "digits only" check happily passes "abc123abc" because it found the digits in the middle. Anchor both ends — start-of-string and end-of-string markers — when you mean the whole string must match. In Regex Lab, toggle the multiline flag and watch what the anchors bind to change.
Escaping. A literal dot is not a dot in a pattern; unescaped it matches any character. Backslash-escape the dot, the plus, the parentheses — any character that has a meaning — when you want it literally.
Start from something that works
The built-in library seeds the workbench with known-good patterns — email, URL, IPv4, ISO date, semantic version, duplicate word. Starting from one of those and adjusting it beats writing from a blank box, because you begin from something that already matches and only change what your case needs. The flag chips each carry a note on what they do, and a token cheatsheet sits underneath for the piece you half-remember.